Canadian Airgun Forum

I got a chance this afternoon to make the hammers.... I turned them from 1144 Steel.... They are 1.20" long, drilled 0.50" to a depth of 1.00" to the drill shoulder, and weigh 104 grams each.... There is just enough room in the cocking slot to go from contacting the back of the modded valve I made (which was shortened 0.075") to just cocking the trigger, for a total hammer stroke of 0.88" with my valve.... and about 3/4" with the Cothran valve.... With the deeper recess for the spring I have 2.75" of length from the shoulder in the hole to the back of the tube.... This means I can use a 3" spring for my SSG without having to make the rear plug stick out more than 1" behind the tube or so.... I'm pleased with the way they came out....



You may wonder why I am always using heavy hammers with maximum stroke in my builds.... Most of the time I'm going for a lot of FPE with a heavy bullet, and that means I need a large valve throat and lots of dwell.... The hammer energy to crack the valve and create lift comes from the spring and travel, but the hammer momentum to create more dwell comes from hammer weight.... A heavier hammer allows me to use a lighter spring, with a lower cocking force, and still get the FPE I want....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Today I made the first SSG.... I left all the parts longer than necessary until I find out if it will work properly throughout the range of tuning I need, I can always shorten them up and make them prettier later....



The housing is a piece of 1.25" OD 6061-T6 aluminum turned down to fit in the back of the main tube, with the appropriate tapped holes to secure it.... On the top are two 4-40 screws through the receiver on 0.688" centers, and on the bottom a single 8-32 hole that mounts the trigger group.... The front just clears the back of the hammer when it is all the way back.... It is drilled through and tapped 9/16"-18 NF for the gap adjusting bolt.... I know that is huge, but it allows the spring assembly to be withdrawn out the back without any other disassembly.... That is a feature I always like to employ on my RVAs and SSGs, it makes changes a breeze.... The gap adjusting bolt has a spring seat turned on the front end, to hold the spring in alignment.... The stop rod part of the spring guide is a 4.5" long piece of 3/16" drill rod, threaded 10-32 both ends.... At the back it has quite a bit of thread for preload adjustment, which is provided by the two nuts locked against each other.... There is a # 008 O-ring on the shaft to act as a cushion when the guide comes to a halt.... The front part of the guide is turned from a piece of aluminum rod....It is 0.48" OD at the front (the same as the spring) and profiled to fit the bottom of the hole in the hammer.... Behind that is a 1" long section that is 0.355" OD to act as a spring guide.... There is enough space between the two parts of the guide to allow for more preload and cocking distance than I need.... The spring I am using at the moment is 3" long (instead of the 2.5" used in a stock MRod), with a rate of 13 lb/in.... I have 0.40" of preload at the moment, which is about 5 lbs.... If I need more, I just wind the nuts down on the shaft.... If I need less, I can shorten the gap adjusting bolt (moving the rear spring seat back), but a better solution would be a weaker hammer spring.... I just don't have anything lighter at the moment....



In the photo above, the SSG is adjusted for zero gap with the current 0.40" of preload with my valve.... With the Cothran valve, where the end of the stem sits further back, the gap adjusting bolt would be further back.... I can shorten the back of the SSG housing to allow more adjustment if I have to increase the preload much.... I'm thinking I'm getting pretty close to being able to tether the version with my valve in it.... That will answer a lot of questions about the hammer spring requirement at various pressures.... The Cothran valve will require less hammer strike than mine, at any given pressure, so I could use a lighter spring, or just increase the gap.... My valve at 3000 psi will be the test of the maximum hammer strike required for these guns....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I made the Teflon transfer port today and assembled the gun and fired it for the first time.... Tethered at 1900 psi, at zero gap on the SSG, the velocity was a bit low, so I increased the preload in two 0.10" stages from 0.40" to 0.60", at which point I was able to hit the velocity plateau at zero gap.... I then installed the action in the wooden stock.... It weighs right on 7 lbs. without the bottle.... I haven't done anything with the comb of the stock yet....



I then tested the velocity with 25.4 gr. JSB Kings and 34.2 gr. JSB Heavies, adjusting the Gap from zero out until the velocity dropped below 600 fps.... and then went the other way, adding preload (negative gap) until the plateau was confirmed.... ie I knew what the maximum possible velocity was with this valve, and 0.219" ports, at 1900 psi.... Here are those results....



As expected, adding any preload against the valve stem instantly increased the report.... I like to adjust the preload on my SSGs until I can reach the velocity plateau just as the gap goes to zero.... Any more preload and you are just making the gun harder to cock, and it means that you have to increase the gap to drop the velocity back down to the knee of the curve.... I want just a bit more power, I would like to be on the knee of the curve at about 950 fps with the 34 gr. Heavies, so with this valve I would need to increase the regulator setpoint slightly, I think 2000 psi should be about perfect.... However, before I make that decision, I want to test the other tube with the Cothran valve at the same pressure, to compare the results....

All in all, I'm delighted with the initial results.... With the SSG adjusted so that there is a tiny gap, I am at 944 fps (68 FPE) with the 34.2 gr. JSB Heavies, at only 1900 psi.... Moving the seat forward in the MRod valve so that I was able to achieve the 7/32" exhaust port was worth the effort....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Nice work Bob! Your lathed and machined hammers are way nicer than my drill press versions...

Be nice to see what those bullets look like after firing...

-D.S.

_________________
"Ain't no half-way"
-S.R.V.

The story of todays post is.... NEVER BE AFRAID TO LEARN SOMETHING NEW.... or at least open your eyes to existing problems.... ::)

I am always careful to make sure that the area between the front of the hammer and the back of the valve can't get pressurized, either from the hammer movement itself, or from an air leak through the valve stem during firing.... If that happens, you can get (in the worst case instance of a leaky valve stem) a machine-gun action where the pressure partially recocks the hammer and fires the gun again.... until the reservoir is empty (yes, I've had that happen).... Travis suggested that I flute my hammers to prevent this, but I double checked, and the sear slot in the bottom of the tube vents into the trigger, and out through the bottom.... However, I haven't been paying enough attention to the BACK of the hammer, where a vacuum can build up and slow the hammer strike.... The only gun I ever vented there, on a suggestion from Scott (Motorhead), was my B-51.... WHY I haven't been looking for this problem in other guns I don't know.... complacency, I guess.... Well, that just bit me.... in spades....

While I was triple checking the space in front of the hammer this morning, I looked at the area behind it.... Yes, there is a rather large cocking slot, with a matching one in the receiver.... but guess what.... There is nowhere for the receiver to vent, other than past the bolt at the ends, in an MRod receiver, with the rear bolt.... I had noticed some large ES values yesterday when I had the SSG backed off with a large gap, and the lightbulb finally went on.... With an SSG (or SSS or TSS), the spring quits pushing the hammer before it gets to the valve stem, and it coasts the last part.... What if there is a vacuum behind the hammer?.... Yup, that will start slowing the hammer down before it gets to the valve.... In addition, that vacuum is unlikely to be 100% consistent, shot to shot, which could increase the ES.... and the more gap you have between the spring and hammer, the worse both might be.... Time to find out....

I set the SSG for 1.5 turns of gap, and shot an 8-shot string.... The average velocity was 811 fps, but the ES was huge, with shots going from 790-834 fps.... definitely not good.... I stripped the back of the gun apart, and machined two slots in the bottom of the rear plug (SSG housing), at about 4:30 and 7:30, in the section that inserts into the tube.... The slots were 3/16" wide and 1/8" deep, and continued back 3/16" behind the rear of the tube.... That is the equivalent area of a 1/4" hole through the rear plug to prevent any vacuum forming behind the hammer on firing.... Here is what you see with the action out of the stock....



There happens to be a slot in my stock at the location of the vents.... If that isn't the case with a different stock, a different vent design may be necessary.... Anyway, I assembled the back of the gun again, adjusted the SSG gap to the same, 1.5 turns out, and fired a 34 gr. JSB through the Chrony.... Instead of 811 fps, it was 956 fps.... WOW !!!.... Obviously the vacuum behind the hammer was a reality, I had moved up onto the velocity plateau.... I went through the process of documenting the velocity vs gap with the vented rear cap.... On the graph below, the dotted red line is the new data, with the vent.... while the solid red line is yesterday's data, without the vent....



There are two important things to notice.... First, it takes a LOT more gap to get to the knee of the curve with the vents.... which means that without them, the vacuum behind the hammer was slowing it dramatically.... Secondly, the slope of the downslope part of the curve is a lot shallower.... It takes more gap to lose the same amount of velocity.... To drop from 944 fps down to 760 fps now takes a 3 turn increase in gap instead of two.... That makes sense, because the bigger the gap, in the unvented setup, the more the vacuum would reduce the hammer velocity.... There is absolutely NO doubt this gun needs the area behind the hammer vented....

Since I had dramatically increased the hammer strike at the same settings, I could now do two things.... I could reduce the preload on the spring, making the gun easier to cock.... I reduced the preload to my original 0.40" (which was not enough yesterday, with the vacuum), and shot the data shown by the purple line.... As you can see, the "knee" of the curve still requires 3 turns more gap, even though the preload on the spring is reduced from nearly 8 lbs. to just over 5.... Since the larger gap means the spring is not compressed as far (and starts at less preload as well), the maximum cocking force, when the gun is set just below the plateau, at 944 fps, has dropped from 19 lbs. down to just 15 lbs.... In fact, I could further reduce the preload if I wish, and that would allow me to decrease the gap, and the cocking force, even more....

These vents did one more thing for me.... They increased the available hammer strike to the point that I could now test the gun at 2900 psi.... I checked it with the 34 gr. JSB Heavies, and the results are shown as the black line on the graph above.... This was shot with the preload on the spring still at 0.40", and to get to the plateau I had to set a turn of negative gap (actual preload on the valve stem).... I didn't try it, but I am confident that by going back to the 0.60" preload I could operate at 2900 psi and hit the plateau with a small gap.... In any case, these results proved that this valve is capable of cranking out 90 FPE at 2900 psi with the 34 gr. Heavies.... I didn't have the proper sized 51 gr. BBTs for this barrel, the ones I had were sized to 0.250" (so they were likely leaking a bit in the bore).... but by cranking in some preload I managed to find the plateau for them as well.... I was able to hit 963 fps (105 FPE) at 2900 psi with those bullets.... I have ordered an NOE Mould for the 51 gr. BBTs in 0.253/0.255", and the sizing bushings for them.... so at some point in the future I can experiment with them....

In conclusion, I am extremely glad I vented the area behind the hammer to prevent the vacuum from slowing the hammer.... I am frankly shocked at how much hammer strike was lost because of that vacuum, and the horrible ES that it caused.... You can bet I'm going to have a long, hard look at all my Disco based guns.... There may be more consistency, and easier cocking, lurking amongst any of them.... ::)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

After writing the previous thread, I changed all the parts over to the tube with the Cothran Powerhouse valve in it.... This is the best way to compare it to my modded MRod valve, by tethering it at the same two pressures, 1900 psi and 2900 psi, and using the same pellets.... Here are the results.... The horizontal axis on this graph is the same as the one above for my valve, and I used the same colours for the 34.2 gr. JSB Heavies at the two different pressures.... In both cases, the SSG had the same 0.40" preload, and "Zero" is the point where there is just barely a gap in the SSG setup.... The end of the valve stem in my valve is further forward than that of the Cothran valve, so my setup has 0.88" of maximum hammer stroke, whereas with the Cothran valve it is only 0.79".... but that suits the two valves, because mine needs more hammer energy to open.... This means that the SSG has to be adjusted to a different point (about 1.5 turns further out) with the Cothran valve to end up with zero gap.... but zero on both graphs has the same meaning, minimum gap on the SSG....



You can see at once the huge difference in the response of the Cothran valve to the gap setting.... Basically this valve is either "ON" or "NOT".... It exhibits the typical "cliff" which we have seen in other tests of the Cothran valve.... The most efficient point to operate the Powerhouse valve is just above the point where it no longer opens properly and consistently.... This may only be a matter of one flat (1/6 turn) on the SSG gap adjuster.... I didn't explore the exact location of that point yet, all I was after was the maximum velocity available at the two test pressures, and the range of SSG gap where the cliff started.... Relative to zero gap, at 2900 psi the cliff was at about 2 turns out, compared to requiring actual preload with my valve.... at 1900 psi, the cliff was at about 4 turns out, fairly similar to where the knee was with my valve.... As expected, with the larger ports, the Cothran valve was able to produce a bit more power than my valve was.... Here are the maximum velocity and energy achieved with the two valves at the two test pressures, with the pellet and bullet tested....



The port size on my valve is 0.219".... The Cothran valve has a 0.257" exhaust port, I used a 0.250" transfer port, and the barrel port is equivalent in area, but there is a 3/32" diameter probe in the chamber.... This reduces the effective area to that of a 0.234" port.... so it doesn't hurt my valve, but probably handicaps the performance of the Cothran valve slightly.... However, the only way around that would be to just a probeless bolt with a flat face, that retracts flush with the back of the barrel port in the firing position.... This would require milling a "J-slot" in the MRod receiver for the bolt lockup and cocking pin to retract into.... Since I don't plan on using anything heavier than my 51 gr. BBT, and I can already push that to nearly 1000 fps, I simply don't need to bother making that modification to the receiver.... However, if you wanted to use an even heavier bullet, as you might if you fitted a .257 cal barrel, you would be well advised to take the trouble to use a probeless bolt arrangement to achieve full barrel size porting with the Cothran valve....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I pulled my modded valve out of the tube today and bored out the exhaust port to 0.234", and drilled out the transfer port to that diameter as well.... I inspected the MRod poppet, and it has a nice shiny seat, just indented a few thou.... It is not showing any sign of extruding into the port, as I have seen happen with a 0.281" throat.... The throat in my current valve is 0.266", which I have also used before without issue at 3000 psi.... However, I don't want to go any larger.... Unless you change the poppet material, going larger on the port, or using more than 3000 psi, is just asking for trouble.... The throat (when you subtract the 1/8" stem), the exhaust port, the transfer port, the barrel port and the bore (when you subtract the 3/32" bolt probe) are now all the same area.... I consider this the maximum practical for a modded MRod valve, unless you change the poppet, and no point in doing that unless you also go to a probeless bolt....

The larger ports increased the maximum velocity at 1900 psi by 24 fps, from 956 to 980 fps.... Here is the comparison, before and after enlarging the ports....



I want the ability to achieve 950 fps on the knee of the curve, and with the 0.219" ports I would have needed to increase the regulator setpoint to 2000 psi.... Now I can use 1900 psi and get there.... I adjusted the gap to 3.5 turns out, and shot the following 10-shot string using the 34.2 gr JSB King Heavies, tethered at 1900 psi....

960
959
956
959
959
957
957
960
955
957

ES = 5 fps = 0.5 %
958 fps average
69.7 FPE average

This achieves my goal with these pellets.... I don't have an efficiency yet, that will have to wait until I install the bottle.... I have not yet decided if I will use my valve, or the Cothran, for the regulated .25 cal, either will work perfectly.... The edge for the Cothran valve over mine is now just 10-20 fps at 1900 psi.... depending on how I adjust the SSG gap.... To adjust the velocity on the Cothran valve, you have to change the pressure, as for best efficiency you need to operate it just above the cliff.... It would make sense to use whichever proved to be the most efficient at the velocity I end up shooting.... That has yet to be determined....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I've been hearing all about these MDS Nylon lightweight hammers, and this gun, with the current hammer spring, needed a large gap on the SSG, so it seemed a good candidate to try one.... I made one of identical dimensions to the steel one I had, but used a 3/4" diam. steel core and the rest is MDS.... I simply knurled a piece of 3/4" steel, drilled it for the hammer spring, and then bored out a piece of MDS of the correct length to be a press fit, and pressed them together.... I then machined the MDS to the correct diameter, cut off the excess steel at the front and turned it to length, and drilled and tapped the 8-32 hole for the cocking pin through both, then machined the angle for the sear in the MDS.... The finished weight was 51 grams, almost exactly 50% of the original steel one, which was 104 gr.... I installed it in the tube with my valve in it, and tethered the gun at 1900 psi, to duplicate the test I did yesterday of the 0.234" ports.... I used the same 34.2 gr. JSB Heavy pellets, so the ONLY change was the hammer.... Here are the results....



With zero gap, the velocity was already starting to fall below the maximum, at 963 fps.... This is virtually identical to the velocity I got with the heavy hammer at 3 turns of gap.... The velocity at 1 turn out with the light hammer was almost the same as at 4 turns out with the heavy hammer as well.... Therefore, reducing the hammer weight by 50% required a 3 turn decrease in gap to obtain the same velocity.... The hammer spring I am using has a rate of 13 lb/in, and 3 turns of the gap adjuster, which is 18 TPI, is 0.165".... That means that the light hammer requires just over 2 lbs. more force to cock the gun, and 0.165" more hammer travel, to obtain the same results....

Using Lloyd's hammer spreadsheet, I calculated the hammer velocity at the instant it left the SSG, and from that the hammer energy and momentum.... To do that, I had to include the weight of the SSG guide (22 grams) during acceleration, to get the velocity, and then subtract it to get the energy and momentum of the hammers.... Here are the results that produced identical (963 fps) velocity and FPE in the pellet, which is what matters, of course....

104 gr. Hammer
Velocity 11.54 fps
Energy 0.471 ft.lb.
Momentum 0.082 ft.lb/sec
Lock time 10.1 mSec
Cocking Force 14.4 lbs.

51 gr. Hammer
Velocity 17.83 fps
Energy 0.554 ft.lb.
Momentum 0.062 ft.lb/sec
Lock Time 7.1 mSec
Cocking Force 16.6 lbs.

These results are skewed somewhat by the admittedly heavy SSG assembly in this gun.... as it represents a much higher percentage of the weight of the light hammer than the heavy one.... However, in this particular application, that's what I got.... There is no question that this lightweight hammer will work in this gun, with either valve, at 1900 psi with the 34 gr. JSBs, it would just be a matter of tuning it for the velocity you want.... There is also no question that it requires about a 15% increase in cocking force to get the same performance.... I expected an increase, but it's nice to actually put some numbers to it.... Bear in mind, this is a pretty powerful .25 cal, delivering 70 FPE at only 1900 psi....

Although I did not try it at 2900 psi, the results I could expect were pretty obvious.... I was unable to max. the velocity out with the 34 gr. JSBs at that pressure with the heavy hammer and 0.40" of preload on the spring.... I think I could just get there with 0.60" of preload at zero gap, which would put the cocking force at about 19 lbs.... With the light hammer, I'm pretty sure I couldn't come close to maxing the velocity using an SSG, I would have to go to a preloaded spring.... Using heavier bullets at 2900 psi it would be even harder to flow enough air to max. them out, even with the heavy hammer.... Even the Cothran valve had to have the SSG dialed down to zero gap with the heavy hammer, to get to maximum velocity with my 51 gr. BBTs....

My conclusion is that the MDS hammers certainly have their place, and can do wonderful things at moderate power and pressures.... However, when you are trying to extract maximum FPE, particularly with heavy bullets and at high pressure.... a heavy hammer helps you get the hammer strike required without using a spring that make the gun too hard to cock.... The Cothran Powerhouse valve, with its ability to open with a much lighter hammer strike, can certainly make use of lighter hammers, however.... I have done no efficiency testing with either hammer or valve, so can draw no conclusions regarding that at this time....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Today I hooked the gun up to a regulated 500 cc tank so that I could take pressure readings on that tank as I shot some short strings to check the efficiency.... I started with the tube with my valve in it, with the MDS hammer.... I adjusted the SSG to zero gap, shot one 8-shot magazine using the 34.2 gr. JSB Heavies, and recorded the average velocity and the pressure drop.... I then turned the SSG gap adjuster out one turn, repeated, and then did the same thing at 2 turns of gap.... I then changed to the 25.4 gr. Kings, shot another magazine with those, and continued, using the lighter pellets, for 2 more turns.... I then replaced the MDS hammer with the steel one, and starting from 4 turns out (where the starting velocity was the same, 945 fps with the 34 gr.), kept going, repeating the above with the steel hammer.... Then I calculated the FPE/CI based on each pressure drop and the total FPE of each string.... and plotted all the data as below....



The efficiency is very similar with the steel and MDS hammers.... The gauge I was using to monitor the 500 cc tank had 100 psi increments, so all pressure reading are only within about 10 psi at best, but I feel that the trends above are valid, even if the exact values are not perfect because of the 8 shot strings, which resulted in pressure drops from 120 to 240 psi over the 8 shots.... More accurate numbers would require longer strings..... I also averaged the FPE/CI values at the end of the 34 gr. string and the beginning of the 25 gr. string, because they were pretty close (the heavy pellets were a bit better).... The important numbers, of course, are what the efficiency is at the highest velocities tested (945 fps with the 34 gr. JSBs).... 8 shots used 240 psi with the MDS hammer and 230 psi with the steel hammer, so you can call it a wash (that's as close as I can read the gauge).... That works out to 1.08 - 1.12 FPE/CI at 68 FPE.... and that means I can get about 38 shots per tank from 3000 psi down to the 1850 psi setpoint I was using....

I then swapped tubes, fitting the one with the Cothran valve and the steel hammer.... I had to back the SSG wayyyyyyyyy out to find the cliff, because the last time I used that valve, I was getting a vacuum behind the hammer, which I have since cured.... With the steel hammer, the cliff occurred at just over 8 turns of gap on the SSG, and with the MDS hammer at just over 6 turns.... I followed the same procedure as above, but only using the 34 gr. pellets, with the following results....



I did not record the pressure drop, or calculate the efficiency, after the velocity fell off the cliff, as there was no point, you would never operate the gun there.... I did find that the difference between the valve opening properly and intermittently (fast and slow shots mixed) was only one flat (1/6 turn) with the MDS hammer, and then 1 more flat put the velocity in the basement.... With the steel hammer, just 1 flat went from full power over the cliff.... You can see that as you approach the cliff from the high-velocity end, the efficiency rapidly increases, particularly over the last turn.... A couple of turns too much on the SG, and the efficiency is a dreadful 0.45 FPE/CI, with the gun using about 80 psi per shot.... The best efficiency occurs just above the cliff.... For 8 shots, the best I could do was 290 psi drop with the MDS hammer and 310 psi with the steel hammer.... Again, those are close enough that there may not be any real advantage to either hammer.... It works out to 0.88-0.94 FPE/CI, which is significantly less than I got with my modded valve.... That works out to 30-31 shots per tank, compared to 38 with my valve.... Note, however, part of that will very likely be the fact that the velocity with the Cothran valve was greater.... 970 fps (72 FPE) compared to 945 fps (68 FPE) for my valve.... If they were tuned to the same velocity, the difference in efficiency may well disappear.... However, that would require running the Cothran valve at a lower pressure than my valve.... likely around 1700 psi instead of 1850, to drop it down to 68 FPE.... That would increase the headroom (pressure difference between fill pressure and setpoint) from 1150 psi to 1300, which in itself would raise the shot count to 34-35 shots per fill.... If there was any increase in efficiency, it wouldn't take much to get back to the 38 shots from my valve.... In all fairness, I would have to say that the efficiency of the two valves is VERY similar.... so much so that a tiny tweak one way or the other could make either valve come out on top....

There is another variable that will likely effect the ultimate FPE/CI numbers.... For all versions except my valve with the MDS hammer, there was a lot of gap in the SSG.... A better solution would be to fit a weaker hammer spring to reduce the gap.... Small gaps generally make for narrower ES values.... I would use the MDS hammer with the Cothran valve, in addition to a lighter spring.... the valve takes that little to open it.... One other thing that could be tried with the Cothran valve would be to artificially limit the lift of the valve by using a bumper on the back of the valve, and an adjustable, recessed hammer face, like Lloyd was experimenting with.... It seems to me that the Cothran valve, as is, simply releases too much air to be efficient in .25 cal. with pellets.... To get the velocity low enough you have to drop the setpoint pressure down a long ways.... I have a gut feeling that using a higher pressure, say 2000 psi instead of 1700, and then limiting how far the valve can open with a stop, may increase the FPE/CI.... I hope Lloyd continues his experiments with that at some point.... Using my valve, I think bumping the pressure up a bit, back to 1900 psi, or maybe even 2000.... and then operating the valve further down on the knee.... will improve the efficiency of that as well.... and may well buy me a few shots below the setpoint, giving me a similar shot count to what I have now, at my 70 FPE goal....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Today I moved back the port in the .30 cal barrel that I had to line up with the port in the MRod receiver.... I was able to get everything to line up without having to cut another O-ring groove, shorten the barrel and machine a new chamber, so that saved me a LOT of time.... The new barrel port is 0.22" wide by 0.28" long, so it is the same area as the 1/4" transfer port.... and the new spot face around the port is 3/8" to match the receiver, valve, and the OD of the transfer port.... The barrel was already machined for a Hatsan Air Stripper, so with just the change in port location it was ready to fit with a Carbon Fibre sleeve to stiffen it.... The barrel is a TJ's with a 1/2" OD and I had some high modulus CF tubing of that ID that was 16mm OD, the same as the .25 cal LW barrel I used, which I had already machined the receivers to fit.... By insetting the CF tube into the receiver, there is no weak area at the front of the receiver for the barrel to bend, and the high-modulus CF the tube is made of is 50% stiffer than normal CF, which puts it at least as stiff as a steel barrel of the same diameter.... Instead of being able to flex the barrel with your hands, once sleeved, it's like a rock....

I sanded the barrel OD and the ID of the tube with 220 grit to give a bit of "tooth" for the adhesive.... I used Loctite 638 (green) which is high-strength, high-viscosity, intended as a retaining compound for slip-fitted parts with a gap of 0.010" or less.... It has a shear strength of 4500 psi once cured, stronger than most epoxies, has a working time of 4 minutes (or less) and a curing time of 24 hours.... Once cured, I will be giving it a post cure in my wife's oven at 175*F for 3 hours to assure full strength.... You MUST assure that the parts slide together nice and easy, if they stick and you have to force them, you may run out of time, because the tighter the fit, the quicker 638 sets to the point you can't move it.... I used two nuts on the threaded muzzle portion, and an O-ring, as a stop for the sleeve, to insure proper placement.... but a few wraps of masking tape will also work just fine.... Make sure you have the CF tube exactly the length you want, and know where you want to position it, before you apply any glue.... Make sure you plug the ends of the barrel and any ports, with wadded up paper towel, etc., to keep the glue out.... Here is what it looks like with the CF tube glued in place....



The portion of CF tube in the receiver is about the same length as the steel part where the port is.... I had to drill out the cone of the Hatsan Stripper to 11/32" to have ample clearance for the .30 cal bullets.... This one started out drilled for a .177 cal.... The other job I did today was to change the cocking and lockup pin in the bolt to 10-32 for safety with the extra loads from the .30 cal.... At the same pressure, the force pushing the bolt back is 44% greater than on a .25 cal, and going up from 8-32 to 10-32 increases the shear area by 45%.... In the .30 cal, at 3000 psi, there is 212 lbs. force on the bolt, and although the stock 8-32 should be able to handle that, if you ever fired the gun without having the bolt handle down, that force would accelerate the bolt backwards, with only that 8-32 bolt to stop it before it hits your face.... We have never heard of Crosman having a problem, but the MRod only goes to .25 cal from the factory.... Using a 10-32 gives me the same safety margin with my .30 cal.... Here is a photo of the receiver showing the new pin and the wider slots for it....



The MRod bolt is case hardened, so the first thing I had to do was heat it red hot around the threaded hole for the pin, and from there forward.... By letting it cool slowly, it anneals the steel, making it much easier to machine.... although there is still a bit of a tough skin.... I carefully lined up the 8-32 hole with my lathe chuck, using a long screw in the chuck, and clamped down the bolt.... I then removed the screw from the chuck, and drilled the hole out and tapped it 10-32.... One tap I had was a bit older, and I thought I might break it, as the metal was still hard to thread.... but my other tap, a nice new one, cut the threads no problem.... I then used a 1/2" long 10-32 SHCS, and loctited on a hex nut, tightened hard against the head of the screw.... I turned the nut, and the head of the screw down to 1/4" OD, and shortened the head about 0.025" so that it was just below flush in the receiver when installed.... I had to mill out the cocking slot in the receiver wider as well, along with the cross-slot on the left side (I will only be installing the handle on the right) so that the new, larger locking pin would fit....

The last thing I did today was to grind off the probe and the nose of the bolt, leaving a 1/4" stub about 3/4" long.... I will make a new .30 cal nose with probe that will be slid over the stub and loctited in place to complete the conversion of the receiver to .30 cal....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I finished up the .30 cal Bolt conversion today by increasing the diameter of the forward section.... Now you can clearly see the larger cocking / locking lug....



I ground off the probe, leaving a straight 1/4" diameter shank 3/4" long.... and then made a sleeve to slide over the 1/4" diameter shank from a piece of 5/16" drill rod.... I turned the front part down to form a probe that is 7/64" in diameter, which just fits inside the base of the JSB 45 gr. and 50 gr. pellets.... The flow area remaining around the probe is the equivalent of a 0.280" port, and the ports in the barrel and Cothran valve are only 1/4", so it should not restrict the flow.... I turned down (filed and sanded, actually, in the lathe) the OD so that it would just slide into the chamber of my .30 cal barrel, checked the alignment, and glued it onto the stub of the bolt with Loctite 638 (green) and allowed it to set....

The shoulder on the bolt nose is located so that the outside edge of it is flush with the back of the barrel port.... and the probe will just push the skirt of the JSB pellets past the front of the barrel port.... When you are making a longer, thinner probe, you are limited in the overall length by how far it will withdraw to clear the back of the magazine.... With the hammer and cocking pin in place, I temporarily install the receiver, with the bolt in place, and pull it back as far as the hammer will allow it to go.... and then mark and cut the probe off flush with the back of the magazine notch.... That will still be a bit long, once you install the trigger, the hammer when cocked will sit slightly forward of that point, and you may have to shorten it a bit more.... The problem is, that if you do, it may not push pellets past the longer barrel port, so you may be stuck with having to hold the bolt back to remove the magazine.... On my .25 cal, I am just able to remove the mag. with the gun cocked, I won't know on this one until the final assembly.... Worst case, I don't mind holding the bolt back to remove the mag. if that is the only way to get the pellet past the barrel port.... Like with many conversions, there is often just enough (or not quite enough) room for things like this....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I assembled the .30 cal version today, using the tube with the Cothran valve and the MDS hammer, and did some test shots tethered at 1900 psi and also at 2900 psi.... After about 20 shots at 2900 I started to get some blowby, and within 3 shots, it was severe, with a 200 fps velocity drop.... I pulled the gun apart to find this....



Yep, that's whats left of the Teflon Transfer Port.... Looking at the damage, I think the bottom edge which was thinned out to clear the O-ring in the Cothran valve failed, which let HPA out between the valve and the OD of the port, which then collapsed the port.... Interesting failure, anyway, and the first time I have had a Teflon TP do anything strange.... If there was no O-ring in the Cothran valve, and the TP tube was full thickness to the bottom, I think it would have been fine, as I have used them at much higher FPE and pressure levels.... Anyway, I'll have to come up with a solution, maybe just use PEEK?....

I was quite pleased with the numbers I achieved.... I tested both the 45 gr. and 50 gr. JSB pellets at 1900 psi, and then tested some NOE 67 gr. BBTs and also some Accurate 71 gr. BBTs at both pressures.... Here are the results, with the SSG gap adjusted so that the valve is operating just above the cliff.... NOTE - I EDITED THE TABLE BELOW AFTER MAKING A BRASS TRANSFER PORT - I GUESS THE TEFLON ONE WAS FAILING, BECAUSE I GAINED 50 FPS ON THE 2900 PSI SHOTS....



These represent some pretty impressive power at those pressures.... I now have to make the same tests on the other tube assembly, with the steel hammer and my modded valve.... I got my new NOE .253 cal 51 gr. BBT mould today, so once I get some cast up I will be able to see what these guns will do with those at 2900 psi.... I also need to try the 41 gr. BBTs as well.... so lots of testing ahead yet....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I ran all the tests again, with more bullet weights, and determined the position of the cliff.... As expected, it is a bit different with pellets than with bullets.... The lighter pellets need 1/2 turn less SSG Gap (ie slightly more hammer energy) to stay above the cliff.... Here are those results....



It only takes 1 flat (1/6 turn) on the SSG to go from operating properly to being intermittent, and another flat to see the velocity in the basement.... For the velocity tests with the different bullets I adjusted the SSG so that it was solidly on the plateau.... I added the 62 gr. NOE HollowPoints to the testing today, and also I had some .308 cal 80 gr. BBTs from Accurate that I sized down in 2 step) so that I could try them in this barrel.... All the bullets shot today were sized to 0.300".... Here is the expanded table of results at 1900 and 2900 psi with all bullet weights tested....



Note that the Cothran valve pushes bullets faster at 2900 psi than pellets at 1900.... I didn't do any efficiency testing yet, I'm going to test my modded valve first, putting it through the same tests as I did today with the Cothran....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

I started testing my valve today, and got some data at 1900 psi, but when I changed to 2900 the valve started to leak air out the barrel.... I took a bit more data, figuring it would probably be OK, but the leak got worse so I had to degas the gun and pull it apart.... I lapped the seat and poppet and reassembled, and it still leaked.... I put some soapy water in the exhaust port, and it looked like it was bubbling from the right side, where I had plugged the velocity adjuster hole with epoxy when I inserted the larger, longer seat into the valve when I modded it.... I stripped it apart again, cleaned it thoroughly, and put on some penetrating Loctite, the thin green kind that wicks into threads, hoping it would wick into the cracks and seal them.... I will try it out tomorrow, if it still leaks, I may need to make another valve....

I made an interesting discovery down at my bullet trap this afternoon.... It has several sacrificial layers of particle board, backed by a steel plate, and with all the shooting, it had carved a hole through the layers of wood, down to the steel.... I found these two flattened BBT's laying on the floor, just in front of the trap....



Pretty KEWL, what 150 FPE can do.... *grin*....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

The leak around the insert I made persisted.... so it was time to remake my valve.... I talked to Travis at W.A.R. and he suggested I bore the valve out 1/2" straight through and make a new insert and seal it with O-rings.... Here is what I came up with....



This is a pretty radical repair job, but it gave me the opportunity to do some neat things.... I made the insert a bit longer, to make room for the front O-ring ahead of the port, and enlarged the exhaust port to 1/4" at a 20* angle (same as the 7/32" port was before).... I drilled the throat out to 9/32" to handle the flow of the larger port, and made a PEEK poppet to handle the loads of the larger seat.... This makes the exhaust port basically the same size as the Cothran valve, and as the transfer and barrel ports....

The details of this mod are pretty much explained in the photo above.... The insert is 1/2" OD, sealed with two # 012, 90D O-rings.... There is another hole in the valve for a 10-32 setscrew, in the same plane as the three valve mounting screws, and a dimple in the top of the insert for that to tighten into.... That holds the insert securely in the valve while you machine the port and the three holes for the valve mounting screws.... They are blind holes in the insert, tapped 1/8" deep, using the threads in the valve as a guide, which requires slightly longer valve screws, as the same screws hold the valve and insert in place against the 3000 psi reservoir pressure.... The poppet is made from PEEK, with the stem from 1/8" drill rod, threaded 5-40 into the PEEK.... The stem is 0.070" longer than stock (for my shortened valve).... in a stock length valve it would be even longer....

I assembled the valve and installed it into the tube and it held pressure beautifully.... I then set about testing the .30 cal setup with this new valve.... I had hoped that the PEEK would make up for the larger throat, and not require more hammer strike, but alas that was not the case.... I had to use significant preload at 2900 psi, and a bit of preload at 1900 with bullets.... At 1900 psi with pellets, I had to use preload to max. out the velocity, but the knee of the curve is right about at zero gap on the SSG.... Here is how the gun responds the hammer preload, using the same pressures and pellets as I did for the Cothran valve above.... 6.8 turns out from max. is "zero" gap for the SSG, so at 7 turns and out the SSG is operating, at less than 7 turns there is preload on the valve stem....



When the hammer strike was on the plateau, here are the velocities I achieved with the various pellets and bullets at 1900 and 2900 psi....



Comparing these maximum numbers to the Cothran valve, at 1900 psi there is virtually no difference in fps or FPE.... At 2900 psi, with pellets my valve is almost there with the Cothran, but with bullets I am losing just a bit with the 62-71 gr. bullets and about 10% less FPE with the 80 gr.... I think I may be running out of hammer strike with the heavier bullets, and the difference between the two valves is now insignificant (as you would expect, with equal ports) except for one thing.... My valve is a LOT harder to open, compared to the Cothran.... I'm using a steel hammer that weighs 104 grams, and a lot of preload on the spring, compared to the Cothran, where I'm using an MDS hammer that weighs 51 grams, and lots of gap in the SSG, in fact I could reduce the preload, or fit a lighter spring.... Don's Powerhouse valve sure is EASY to open.... On the other hand, I can tune my valve down onto the knee of the curve, which should make it more efficient.... I think it's time to start trying to work on improving the efficiency of the Cothran valve by making an adjustable lift arrangement so that I can lower the velocity when using the Cothran valve without lowering the pressure....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!