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 Post subject: Re: My Benjamin 392
PostPosted: Mon Jan 20, 2020 11:00 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9075
Location: Coalmont BC
Today I machined off the rear part of the valve at the seat.... Here is the 2-piece valve, ready to install....

Image

The volume between the pieces is 11.9 cc, but by the time I subtract the poppet, spring and spring seat (both parts), it drops to 10.8 cc.... The check valve and spring fill the space in the front section of the valve, so essentially that part of the valve is solid ahead of the rear O-ring.... Here is a chart showing the velocity and pressure vs. the number of pumps....

Image

The pump is a bit more efficient than the version with the double cone that I used with the 6.7 cc valve.... I reached about 1630 psi at 32 pumps filling this 10.8 cc valve.... That resulted in a velocity of 904 fps with the 15.9 gr. pellets (28.9 FPE)…. I tested it at that number of pumps with a couple of heavier pellets.... The 18.1 gr. Heavies hit 870 fps (30.4 FPE) and the 25.4 gr. Monsters reached 770 fps (33.4 FPE)…. I was pretty happy with those results.... The SSG Gap was set at 1 turn to make sure the power was maxed out.... Using this adjustable FTP pump to fill a stock 4.2 cc valve would result in 1600 psi in just 12 pumps.... 8)

I then decided to use just 24 pumps (about 1340 psi), with the 18.1 gr. pellets, and start increasing the SSG gap to conserve more air and see where I could get 2 equal shots, and what the power would be.... Here are those results....

Image

There are some important things to see on that chart.... I can increase the SSG gap to 5 turns before the velocity of the 1st shot starts to decrease.... and adjusted like that there is enough air retained (about 950 psi) for a 2nd shot of over 700 fps and (about 450 psi) for a 3rd shot of over 500 fps.... At 6.5 turns of gap I get two equal shots of 740 fps (22 FPE), and at 7 turns all 3 shots get fairly close together, giving a 3-shot string of 692, 745 and 650 fps.... Unfortunately that is about a 13% ES, which isn't close enough.... so a 3-shot string doesn't seen to be within reach, at least not at over 20 FPE....

The two equal shots at 22 FPE was a bit more than I needed, so I reduced the initial fill to 22 pumps, which is about 1250 psi.... I found the perfect adjustment for the SSG at 7 turns of gap (1/2"), which allows me to shoot a single shot and pump 6 times to refill, or two equal shots and pump 13 times to get back to 1250 psi.... If I needed a 3rd shot for a Coup de Gras, there is enough air for a 600 fps shot remaining after the first two shots.... Here is a sample, with a mixture of single and double shots....

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To say I am pleased with these results would be an understatement.... Not only did I achieve my goal of 2 equal shots, but I actually got 21.2 FPE at 1250 psi.... The gun is not too difficult to pump at that pressure, and it is a lot easier on all the components than 1500 psi would be.... As a retained air pumper, I can shoot continuously at 21 FPE with just 6 pumps to refill between shots.... 8)

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!


Last edited by rsterne on Tue Jan 21, 2020 12:37 am, edited 1 time in total.

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 Post subject: Re: My Benjamin 392
PostPosted: Tue Jan 21, 2020 12:01 am 
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Joined: Sat Nov 05, 2016 10:52 am
Posts: 3206
Location: Caronport, Saskatchewan
Wow great success! Thanks for sharing your progress. Amazing work as always and so well laid out for others to see what was done.

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 Post subject: Re: My Benjamin 392
PostPosted: Tue Jan 21, 2020 8:26 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9075
Location: Coalmont BC
Today I fitted the lightweight hammer that I made the other day.... Now that I have reduced the operating pressure to 1250 psi, it has plenty of adjustment for the 22 pump initial charge.... At that pressure I was getting about 807 fps with the 15.9 gr. pellets (23 FPE)…. With 1 turn of gap on the SSG with the 18.1 gr. Heavies, I got 788 fps (25 FPE), which I'm pretty sure is maximum power at that pressure.... It doesn't really matter anyways, because I will be dialing it down until I get 2 equal shots.... Here is a photo of the two hammers....

Image

The hole for the hammer spring is drilled 0.16" deeper than the stock hammer, so that I can set the SSG for zero gap with the adjusting bolt a turn clear of the back of the hammer group at zero gap, instead of 0.20" behind it.... That shortens how far the SSG protrudes.... The rear of the hammer is about 7/16" OD, and the front portion is only 5/16", which reduced the weight to 40 grams.... In addition, I machined the sear catchment collar on an angle to match the remachined sear, so that the load won't just be on the corner.... I made the hammer out of 1144 stressproof steel, but I didn't bother to harden it.... We'll see how long it lasts.... ;)

I started out at 22 pumps, with the SSG set for 1 turn of gap, and then increased the gap, recording the velocity of all 3 shots, and the pressure after the second shot.... Here are those results....

Image

As was the case the last time I tried this lightweight hammer, the report is noticeably quieter, particularly on the second shot.... The first 2 shots were equal at 3.5 turns of gap (instead of 7 turns), and at that setting the remaining pressure had increased nearly 100 psi with the lighter hammer.... The velocity of the 3rd shot increased from 585 fps with the heavy hammer to 615 fps with the light hammer, proof positive it is saving air.... 8)

It still takes 6 pumps to replace the air used by the first shot, but instead of 7 pumps on the 2nd shot, that has dropped to 6, so to refill after two shots only takes 12 pumps.... This also means that instead of filling to 22 pumps and getting 2 shots, I can just use 16 pumps for the initial fill, and shoot a 21 FPE shot and pump 6 times to refill, and do that for as long as I want to shoot.... ;)

I then repeated the process using 16 pumps as the initial fill (1000 psi), and dropping down to the 14.3 gr. Express pellets.... I started at 5 turns of gap on the SSG, and gradually increased that to 8 turns, recording the results as follows....

Image

The results are similar, just at 16 FPE instead of 21 FPE.... The first 2 shots are equal at 1 flat less than 7 turns of gap, at 712 fps (16.2 FPE) average.... Just like yesterdays results, I can shoot a mixed bag of single or double shots.... It takes 9 pumps to refill after 2 shots, and 4 or 5 after a single shot.... As long as I fill to the high side of the 1000 psi mark on the gauge, all shots are within a 3% ES.... I also have the option of simply filling to 750 psi (12 pumps) and shooting a single shot with a 4 or 5 pumps recharge between shots.... I then repeated the process again, using only 12 pumps for the initial fill (750 psi), again with the 14.3 gr. pellets.... Here are those results....

Image

Two equal shots occur at 2 flats past 9 turns of gap, at 620 fps (12.2 FPE)…. It only take 6 pumps to refill after 2 shots, or just 3 pumps after each shot.... I also found a setting that gave a decent 3-shot string of 585, 611 and 574 fps (11 FPE with a 6% ES) from that 12 shot fill.... It takes 7 pumps to refill to 750 psi to repeat that 3-shot string.... 8)

I tried something interesting at this setting as well.... I pumped just 10 times, took a shot, and then refilled with 3 pumps.... I shot a total of 15 shots like that, averaging 608 fps (11.8 FPE) and the ES over all 15 shots was less than 2%.... This means as a retained air pumper I am getting nearly 12 FPE on just 3 pumps.... 8)

I also checked the efficiency, based on the amount of air pumped into the valve at the three power levels where I was getting 2 equal shots.... At 12.2 FPE I was getting 1.11 FPE/CI, at 16.2 FPE I was getting 1.06 FPE/CI, and at 21.1 FPE I was getting 0.99 FPE/CI, based on the air pumped into the gun.... Calculated using the pressure drop in the valve over those two shots (like you would a PCP) it was between 1.3-1.5 FPE/CI.... :o

So, I can use the heavy hammer and about 30 pumps and hit over 30 FPE with a 25.4 gr. pellet.... Alternately, I can use the light hammer and tune for 2 equal shots of 21 FPE at 22 pumps with 18.1 gr. pellets, or 16.2 FPE at 16 pumps with 14.3 gr. pellets, or 12.2 FPE at 12 pumps with 14.3 gr. pellets.... In any of those three tunes, I can get a single shot at only 16 pumps, 12 pumps, or 9 pumps respectively.... Lastly, I can get a 3-shot tune at 11 FPE filling with 12 pumps, or shoot indefinitely at that setting from a 10 pump fill (just over 600 psi), with 3 pumps per shot giving 11.8 FPE.... 8)

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!


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 Post subject: Re: My Benjamin 392
PostPosted: Tue Jan 21, 2020 10:16 pm 
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Joined: Sat Nov 05, 2016 10:52 am
Posts: 3206
Location: Caronport, Saskatchewan
That's some crazy efficiency, and a lot of great options as well. Very nice!

Sent from my EML-L09 using Tapatalk

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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 23, 2020 6:54 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9075
Location: Coalmont BC
Today I decided to mount a Reflex Sight.... I had a set of Benjamin Intermounts, and they would have worked by placing them against each other, and just ahead of the receiver.... However, the sight line was pretty high using those, and that is right where I hold onto the gun when pumping it.... I also had the identical sight with a Picatinny mount (instead of the dovetails of the intermounts), and a short piece of Picatinny rail left over from another project, just a bit longer than the sight base.... I realized that if I machined a groove in the bottom of that, and screwed it onto the top of the receiver, that I could mount the sight significantly lower, and behind the loading port, where it wouldn't interfere with holding the gun while pumping, and was still far enough forward to miss the bolt handle....

I mounted the piece of rail in the milling attachment in my lathe, and milled it off flat on the bottom and then using a 3/8" end mill I milled a slot that was 0.090" deep.... When placed against the receiver, it sat on the bottom corners of the slot, with the top of the slot just clear of the top of the receiver.... I drilled three mounting holes in it, and milled pockets in the bars between the slots for the head of a 4-40 SHCS.... I then carefully laid out and tapped three matching holes in the top of the brass receiver.... It is 0.150" thick on top, and by shortening the SHCSs to 0.30" I was able to fasten it on securely, and have the screws clear the bolt so that it would still function properly.... It looks like this installed....

Image

The front of the rail is even with the back of the loading port.... The large rear mounting hole in the rail is not used, it was just there on the piece I had left over.... It is actually long enough to mount a small scope, should I decide to do that.... Here is the finished gun with the Reflex Sight mounted and the hole for the gauge stained walnut to match the rest of the stock.... I also shortened the guide rod of the SSG so that it is flush with the preload adjusting nuts, since I will never have to adjust those again.... With the gap adjusting bolt moved forward because of the deeper spring hole in the hammer, you don't even notice the SSG when the gun is cocked, and my hand isn't in a position where it would get pinched on firing....

Image

I grabbed a tin of 15.9 gr. JSB Hades pellets, as I wish to try those on the local Ground Squirrels in the spring.... I fiddled around with the SSG gap and ended right back at 3.5 turns, the same setting as I used for the 18.1 gr. Heavies, which means no tuning is required to switch back and forth between the two pellet types (although a resight may be required)…. I ended up with two equal shots of 765 fps (20.7 FPE) by using 22 pumps (1250 psi) or a single shot at the same velocity using 16 pumps (1000 psi)…. It takes just 6 pumps to refill for each shot fired.... I fired a mix of single and two shots, and over a dozen mixed shots I got an ES of under 3%.... The best consistency (lowest ES) was achieved by filling with 16 pumps, and then using 6 pumps to top up for the next shot.... Most of the shots just a few fps either side of the average, and the lowest and highest shots were only 10 fps apart.... When using 22 pumps and taking 2 shots, the first shot is noticeably quieter.... but the 2nd shot isn't terribly loud either.... about what you would expect for an unsuppressed 20 FPE PCP.... Incidently, while I had the gun apart today I inspected the sear and the hammer, and there was NO wear on either.... ;)

I think I'm pretty well done with this project, and I'm extremely pleased with the results.... There are several different tunes available, everything from this tune and lower just by adjusting the SSG gap to work with the pressure I pump to.... For maximum power, I need to drop the heavy hammer back in, and I can achieve over 30 FPE with the 25.4 gr. JSB Monster pellet at 30 pumps (just over 1500 psi)…. I find that to be a bit too much of a workout for this 71 yo, and the gun is so much easier to pump at 1250 psi, and the performance is great, I'm going to leave it that way.... The flexibility of two shots from that pressure, or a single shot from 1000 psi with just 16 pumps, and only 6 easy pumps to refill is just soooooooo great, I don't see a reason to use anything else.... but the flexibility is there if I want it.... 8)

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!


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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 23, 2020 9:15 pm 
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Joined: Sat Mar 16, 2013 5:15 am
Posts: 3649
Location: Edmonton
Congrats, Bob.
Interesting project.
Incredible results.
Lots of learning.


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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 23, 2020 11:39 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9075
Location: Coalmont BC
That's why I do it.... I learn more on every build.... 8)

I enjoy documenting what I do, not only for the entertainment value for you guys, but to archive it for posterity....

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!


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 Post subject: Re: My Benjamin 392
PostPosted: Fri Jan 24, 2020 1:04 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6887
Location: Rocky Mtn Hse Alberta
Awesome thread here Bob! Love the detail and results. Like looking over your shoulder!
Curious wrt to shooting to POA with the different tensions in the tube. Interested to see what vertical changes you experience.
Keep up the good work eh!

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 Post subject: Re: My Benjamin 392
PostPosted: Fri Jan 24, 2020 4:03 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 9075
Location: Coalmont BC
There is definitely a difference in the POI, even at 20 feet, which is the distance to the backstop in my shop.... When I shoot two shots, starting from 22 pumps, the first one hits a bit higher, even though the velocity of the 2nd shot is the same.... It's not a lot, but enough you can say there are two distinct (but joined) groups high and low, forming a figure 8, rather than one long, vertical group.... I don't know what the difference would be at extended range, or what the difference might be between the lower velocity group (the equivalent of 16 pumps) and shooting at an even lower pressure, but that would also be influenced by reduced velocity as well.... :roll:

I guess this is part of the design, where the brass barrel and brass tube are soldered together.... When you have the load from the pump linkage pushing on the front of the tube, it is bound to stretch a bit, forcing the muzzle up.... :( …. I guess you could prove this by shooting with the pump handle open.... The pressure is only contained inside the valve, which is way back under the back of the barrel.... I don't think that would cause the muzzle to rise much, if at all, I would think it is the force on the front of the tube, transmitted through the pump linkage, causing the "swoop".... :?

This is one more reason that Crosman should have used a mix of Disco parts and a 39X linkage to produce a replacement for the 39Xs, like I did with my .25 cal Disco Carbine....

Image

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!


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