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PostPosted: Wed Dec 17, 2014 4:25 pm 
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Location: Coalmont BC
About 2 years ago I bought a BAM B-51 in .22 cal from sniper when he got a crate of the last of them.... I knew they should be cleaned before shooting them, and I had other projects on the go, so hung it on the wall until "later".... Well, "later" finally arrived this week, so I read "The Book" (which I had previously downloaded), and stripped the barrel and breech off and pulled the hammer out to clean and deburr them and the tube.... The barrel was filthy, and the bolt was very stiff, so I removed the bolt, deburred all the holes and slots in the breech, and cleaned the barrel thoroughly, and gave the breech bore and the barrel a polish with Bore Brite.... The bolt was still stiff and I suspected it was bent, so over to the lathe, chucked the front, and spun it around to watch the handle wobble around nearly 1/16" off center.... No wonder it was stiff.... I spent quite a bit of time straightening it, and that freed it up nicely.... A thin smear of Moly paste and a new O-ring (the original was shredded from the sharp edges I had to remove) and it was ready to reinstall....

I had read about the hammer, and decided that the most suitable mod from "The Book" was to remove the threaded striker, cut a slot in the back end so that it could be adjusted with a long thin screwdriver through the rear plug, and reinstall it, being careful to put it back to the factory length.... I also put a small piece of plastic under the locking screw for the striker, and tightened the locking screw up to compress it against the threads to provide a "brake" so that the striker won't "self-adjust" and yet I can change the setting from the back.... The hammer is VERY heavy, and has a heavy steel spring shim and guide (top hat) inside, with the total weight being 148 grams, of which the top hat is 38.... This is extremely heavy, and is compensated for in the design by having a stiff spring and very short travel.... The normal method of tuning is to screw in the striker from the back (clockwise from the rear) which makes it protrude even further, shortening the stroke even more.... This moves the hammer back, increasing the preload on the spring, and making the cocking stroke shorter but even harder.... I wasn't too sure about the whole concept, but decided that I needed to test it in stock form and make the normal adjustments to get a baseline for further experiments....

I reinstalled the hammer, leaving the safety off so that I could adjust the hammer travel through the end plug with a long thin screwdriver.... The first tests were conducted at the stock travel setting, filling to 1000 psi for a shot, then 1500, then 2000 (velocity increasing), then 2500, 2800, and finally 3000, which resulted in the highest velocity (938 fps) with JSB Express 14.3 gr. pellets.... So, I knew I was in for a falling velocity shot string, at least with the lighter pellets.... I hooked up my Chrony to my Netbook using Chrony Connect, and starter shooting strings.... I then saved them into Excel to create the following graphs....

Image

The initial tests, at stock hammer travel, showed a pretty powerful gun, 27-30 FPE (heavy pellets giving more FPE, as usual), but only 17-19 shots within my normal 4% ES.... The efficiency wasn't bad, considering the way the gun was tuned, from 0.89-0.95 FPE/CI, with the heaviest pellet having the best efficiency.... So far, no surprises, including the excess hammer strike, I expected that from what I had read about the gun, it was tuned for maximum velocity from the factory, not a proper bell-curve.... I removed the stock and cranked the striker in three turns, grabbed the middle-weight JSB 15.9 gr. Exact pellets, and found out I had gone too far.... The velocity at 3000 psi fell to the mid 700s, and took a dozen shots before it got to within 4% of what ended up being the peak at 874 fps.... After recording the string, I turned the striker out a turn, repeated the process, then once more, and the four strings with the 15.9 gr Exacts are all shown on the second graph above.... Here is a summary....

Stock setting.... decreasing string from 909 fps, averaging 894 fps (28.2 FPE), 17 shots at 0.90 FPE/CI, 3000 psi down to 2320....
In 1 turn.... normal bell-curve, 884-909-873, averaging 895 fps (28.3 FPE), 24 shots at 1.19 FPE/CI, 3000 psi down to 2280....
In 2 turns.... normal bell-curve, 862-896-861, averaging 878 fps (27.2 FPE), 25 shots at 1.20 FPE/CI, 2900 psi down to 2180....
In 3 turns.... normal bell-curve, 840-872-842, averaging 861 fps (26.2 FPE), 28 shots at 1.36 FPE/CI, 2700 psi down to 1980....

This confirmed that the factory hammer strike was too much, turning it out a turn gave more power and more shots at greater efficiency.... After that, turning the adjuster in more just dropped the pressure range and power.... The "proper" setting would be about 2 turns out from stock, using the factory rated 200 bar (2900 psi) fill pressure.... My gun was leaking down slowly during testing, so in fact the efficiency is actually better than what I recorded above.... I did NOT like the feel of cocking the gun at 3 turns out on the striker, and it was barely acceptable at 2.... The bolt pulls back a long way before engaging the hammer, and the cocking stroke is short and requires a very hard pull at that setting.... I think I will concentrate on lengthening the hammer stroke, reducing the preload, and reducing the hammer weight for the first round of mods.... The latter two will be done by replacing the top hat with a shorter unit (to reduce the preload) made from Delrin (to lighten it).... I will likely also fit a preload adjuster into the end cap to revert the adjustment to what I consider to be superior to the current setup.... Here is a photo of the gun, with a Leapers 3-9 x 32AO scope installed....

Image

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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PostPosted: Wed Dec 17, 2014 5:33 pm 
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Finally, this is very exciting. I suppose you are planning to go for maximum power and efficiency which of course is natural, however if in your analysis you could project what could be done from 12ftlbs. and then up or to what power level you feel best suits this rifle for that matter it would be great reading for us low power shooters as well. Sorry about the bent bolt, my B50 was in very good condition and took several months before developing an air leak which was brought on by the filter screen self destructing.

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PostPosted: Wed Dec 17, 2014 7:56 pm 
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There is a lot of information in "The Book" about 12 FPE tunes for .177, and that isn't really where my interest lies.... I haven't really decided where this tune is going yet, or whether there might even be a caliber change in the future.... Until I get to know the gun a little better it is pretty much a clean slate....

Bob

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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!


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PostPosted: Wed Dec 17, 2014 9:14 pm 
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Fun rifle, I found a hammer spring at CT that is a little shorter about the same gauge saves cutting the oem one. It took some of the pressure off the stem; might be the cause of the leak. Having said that mine cracked off two days in, at which point I discoverd the valve body is drilled and tapped in place. There was some swarth that got trapped between the body and air tube and scratched the body. not badly but would suggest removing the cap(with seal) before knocking it out. I found a lighter spring on the trigger freed up the bolt, being careful to still be safe. More so then a lighter hammer spring alot of drag on the hammer
Have you slugged the barrel yet mine was quite loose with 5.52.
http://www.airgun.co.uk/Adjusting_and_M ... igger.html


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PostPosted: Thu Dec 18, 2014 1:06 am 
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In this photo you can see the stock hammer, guide, spring, and end cap.... Notice that the striker is sticking out about 2 turns from the nose of the hammer (stock setting was 0.75mm), and also notice the distance from the front of the hammer to the lock screw....

Image

I shortened the front of the hammer by 2mm and made a flush striker to increase the hammer throw by 2.75mm (0.108") over stock.... I also made a very light plastic spring guide that adds only 1/4" of preload to the spring instead of 1/2".... Here are those parts....

Image

I also removed the screw that prevents the hammer from rotating.... It is only necessary if you want to adjust the striker (I don't) and with it removed the hammer can be withdrawn out the back of the tube by lifting up the trigger sear without having to remove the breech.... I reassembled the gun, hoping that the extra hammer travel would mean it would still work about the same with less preload.... It didn't.... I had to put 4 washers under the back end of the hammer spring, and shorten the spring guide because it hit them.... I ended up making the guide the same length as the guide portion on the original metal one because I ended up needing more than stock preload with the much lighter guide.... In other words, I lost more hammer strike by reducing the weight than I gained by increasing the travel.... I shot strings with both 4 and 5 washers, and then put back the original metal guide with no washers, so I had stock preload minus 2.75mm but the spring was still at the same length when cocked as a stock gun.... This combination actually increased the power over stock.... The shot strings, using 15.9 gr. JSB Exacts, are shown below....

Image

The interesting thing is that I gained in efficiency and shot count with the additional hammer travel.... In addition, I ended up with more power when I went back to the stock metal guide (top hat) but with the extra 2.75mm of hammer travel.... Strangely, I also ended up with a bell-curve with that setup, even though the hammer strike was greater than before (when I didn't have one).... Here is a summary of the three strings....

Light Guide, 4 washers.... normal bell-curve, 856-892-856, averaging 878 fps (27.2 FPE), 32 shots at 1.41 FPE/CI, 2900 psi down to 2140....
Light Guide, 5 washers.... normal bell-curve, 870-905-871, averaging 889 fps (27.9 FPE), 28 shots at 1.29 FPE/CI, 3000 psi down to 2270....
Heavy Guide, stock preload.... bell-curve missing some shots at the beginning, 903-924-887, averaging 908 fps (29.1 FPE), 20 shots at 1.27 FPE/CI, 3000 psi down to 2420....

The first string is directly comparable to the first tests at 2 turns out, same power (27.2 FPE), but now 32 shots instead of 25, 1.41 FPE/CI instead of 1.20.... It looks like the additional hammer travel is worthwhile.... I need to fit a preload adjuster to the rear end cap and do some more tests, so that is next....

Bob

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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!


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PostPosted: Thu Dec 18, 2014 8:56 am 
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I wonder why the increased travel equates to better efficiency. Perhaps the factory setup was bouncing?


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PostPosted: Thu Dec 18, 2014 12:19 pm 
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I suspect the valve opens faster with the greater travel as the hammer will be moving faster on impact, but that's just a guess.... I'm still pondering why they use such a heavy hammer and short travel....

Bob

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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!


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PostPosted: Thu Dec 18, 2014 1:33 pm 
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wouldn't that make sense Bob, being a short travel in order to open the valve enough you would need a heaver hammer I would think , if it had a lighter hammer there would be less impact, it has to push against what 3000psi. I would think shorter travel would make much better sense for any kind of reduced vibration, less chance of bounce and maybe improved accuracy, not sure just saying.

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Last edited by Ace on Thu Dec 18, 2014 1:42 pm, edited 1 time in total.

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PostPosted: Thu Dec 18, 2014 1:38 pm 
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Location: Vancouver, BC
rsterne wrote:
I suspect the valve opens faster with the greater travel as the hammer will be moving faster on impact, but that's just a guess.... I'm still pondering why they use such a heavy hammer and short travel....

Bob


I find it amazing that you are able to increase overall efficiency by using your educated guess work, which seems to surpass what their own engineers are capable of. Good work on that.

I'd love to see how the accuracy is with that gun, if you have the chance.

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PostPosted: Thu Dec 18, 2014 6:09 pm 
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There are many solutions for hammers and springs that will work, from light with lots of travel to heavy with short travel, and from stiff short springs with little preload to long soft springs with lots of preload.... As long as the combination has enough energy when the hammer impacts the valve to open it far enough against the reservoir pressure, and enough momentum to give it enough dwell to allow enough air to flow, it will work....

Bob

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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!


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PostPosted: Thu Dec 18, 2014 6:51 pm 
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rsterne wrote:
There are many solutions for hammers and springs that will work, from light with lots of travel to heavy with short travel, and from stiff short springs with little preload to long soft springs with lots of preload.... As long as the combination has enough energy when the hammer impacts the valve to open it far enough against the reservoir pressure, and enough momentum to give it enough dwell to allow enough air to flow, it will work....

Bob


Aah the choices that there are.....lol. :lol:

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PostPosted: Thu Dec 18, 2014 9:51 pm 
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This afternoon I made an RVA (rear velocity adjuster / power adjuster / preload adjuster) and shortened the spring guide to compensate for it.... Here are the parts....

Image

On the left is the guide, shortened 1/4".... In the middle is the B-51 end cap.... I milled a 3/8" diameter recess into it leaving 1/4" of the threads remaining, and then milled a 1/8" slot on the bottom side of the spring seat area.... On the right is the 8-32 adjusting screw, with a thinned nut and the adjuster with pin to prevent rotation.... You insert the screw through the threaded hole in the end cap (it just clears and can rotate) and then install the nut and tighten it against the shank of the screw, which leaves just enough space the screw can rotate but only have about 0.010" end float.... Then screw on the adjuster with the pin the slot in the end cap so it can't rotate....

Image

Once assembled, there is 8 turns (1/4") of adjustment, and the only modification necessary on the stock was to file a small angled notch to clear the ball-end hex driver used to adjust it.... In the photo, it is roughly in the middle of its adjustment range.... When fully out, the adjuster takes up 0.10" of room in the spring seat, so I shortened the guide 1/4" to allow adjustment in both directions from the stock preload.... It is at the stock setting when 3 turns out from maximum preload.... I then tethered the gun to a tank through a regulator set to 200 bar (2900 psi) and shot 5 shots at each setting (1 turn increments) and plotted the average velocity as shown in the top graph below....

Image

Since the stock preload was 3 turns out, and it was on the lower end of the knee of the velocity curve at 200 bar, I thought that would be a good place to shoot my first string.... The results are in the bottom graph above.... I was extremely pleased, as the first shot at 3000 psi ended up exactly 4% below the peak of 904 fps, and I got 32 shots averaging 888 fps (27.9 FPE) from 3000 psi down to 2220, which works out to 1.38 FPE/CI.... That is a great number at that power, one that few PCPs can match in my experience.... The question is why is this gun so efficient?....

There are a few things I noticed that are consistent with many of the things in "The Book".... First of all the initial, upwards part of the shot strings are rather ragged, with significant shot to shot variation (although within the 4% ES), compared to the downward, low pressure side of the string.... Secondly, the pressure drop is almost perfectly linear instead of curved upwards in the middle, which is normal for unregulated PCPs (they usually use more air per shot at low pressure).... Third, I noticed no "burping" and very little increase in the volume of the muzzle blast towards the end of the string.... Fourth, the sear has quite a strong spring so that it drags on the hammer to a significant degree.... Here is what I think is happening.... I think the heavily sprung sear is dragging so hard on the heavy, slow moving hammer, that that friction prevents it from bouncing and wasting air at the low pressure end of the string, and indeed you can't hear any resulting "burrpppp" from hammer bounce.... However, this drag, being not completely identical shot to shot, causes the velocity swings in the first part of the string, exactly where you would expect to see that effect exaggerated.... On the latter part of the string, even large changes in hammer strike will not change the velocity, and we don't see any.... The (almost complete?) lack of hammer bounce shows up in reduced air usage and greater efficiency towards the low pressure end of the string.... I think a lot of that is due to the (what we would think was excessive) sear drag on the hammer....

Now I have a problem.... Should I leave well enough alone and be happy with a fairly powerful and very efficient gun?.... or should I hog out the tiny ports and see if there is a beast sleeping inside this gentle giant?.... I think we all know the answer to that.... *grin*....

Bob

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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!


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PostPosted: Thu Dec 18, 2014 10:46 pm 
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Hi Bob, I'm trying to visualize how you tighten the nut on the rva bolt. Is there clearance for a socket in there? If I understand correctly the spring force is transmitted through the adjuster and bears on the nut - doesn't that make it likely that the nut will come loose as you turn the adjuster?

The sear as anti-bounce device is an interesting theory. I'd never thought there might be an upside to sear drag!

Looking forward to seeing where you go with this, as always.

Regards,
Jim


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PostPosted: Thu Dec 18, 2014 11:09 pm 
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The nut is locked against the shank of the screw where the threads end, and never touches the adjuster.... When fully out, the adjuster sits against the shoulder in the end plug, clearing the nut by about 0.015", so the nut just spins in the hole when you turn the head of the screw, it's only purpose is to keep the screw from backing out the end of the plug.... All I did was jam the point of a small flat bladed screwdriver between a flat on the nut and the inside of the hole it runs in and tightened the screw until the nut jammed against the end of the threads.... If it ever comes loose, I'll put a drop of Loctite or crazy-glue on it, but there is really no turning load from the spring, that is taken by the pin and slot on the adjuster.... Fiddly to make, but it works slick as snot....

Bob

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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!


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PostPosted: Fri Dec 19, 2014 4:14 pm 
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Location: P.G. B.C.
Bob- I really appreciate the work you've done and are doing to this gun. I have one of those rifles, in .22. It's a solid 24.5 to 25fpe rifle with 14 to 16gr. pellets and quite accurate for the first 25 shots before they start to wander left and high. I need to do some more shooting with it as well, might take it to the Island next fall when I visit Rick.
As far as the technicalities go, you lost me somewhere in the first post but thanks a bunch, anyway. I stop reading when the words all meld together :rolleyes: and I get dizzy and fall off my chair. :lol: It's OK, the floor isn't far away. TKS. You're great! I mean it. Love this stuff.

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