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PostPosted: Thu May 31, 2018 9:24 am 
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Full spring = 735fps
Remove 1 coil = 735fps
Remove 1.5 coils (2.5 total) = 315fps

Guess I'm going to have to shim the spring.

I won't bother checking efficiency until I get it in the 700-730fps range. I don't mind giving up some power for a quieter air rifle.

It's definitely not leaking, I've had good luck in that respect. Even with 1 coil removed it still sounded louder than it should. I'll order the parts to check my psi, I agree that it is probably lower than 1400 or it wouldn't be THAT oversprung.


Last edited by 987654321 on Thu May 31, 2018 9:44 am, edited 1 time in total.

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PostPosted: Thu May 31, 2018 9:43 am 
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Must be a leak somewhere. At least it's easy to shim with that spring guide.

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PostPosted: Thu May 31, 2018 11:27 am 
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The fact that the velocity remained unchanged when you removed 1 coil shows you were on the plateau.... Had you checked the efficiency (air use) at that setting, you would have found an increase in shot count, possibly a dramatic one.... Your best combination of power and efficiency should occur at 3-5% lower velocity than the plateau.... ie about 700-710 fps....

Bob

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PostPosted: Thu May 31, 2018 12:16 pm 
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I find this very interesting that the hammer spring is so overpowered that it is still wasting air at 1400psi. If it's true that this lack of efficiency is solely from hammer bounce and too much opening of the valve, it shows how poor the efficiency must be at co2 pressures of 850psi. A person should then be able to double the shot count on a stock co2 gun by simply cutting a few coils on the hammer spring! You'd think after so many years of production Beeman would have corrected this problem.
I still think that along with the hammer bounce there could be a leak, since the efficiency is so terrible. It would at least be worth checking out.

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PostPosted: Thu May 31, 2018 3:01 pm 
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The QB78 typically peaks in velocity at around 90*F, which is about 1200 psi.... or at least it used to before Beeman took it over.... Perhaps they wanted to make sure it would gain velocity at 100*F, and replaced the hammer spring with a stronger one.... of simply didn't know better.... but at room temperature virtually all CO2 guns are oversprung, and will get a huge increase in shot count (sometimes double or more) by shortening the hammer spring....

You may have been on the verge of the velocity decreasing when you removed 1 coil from the spring.... Look at how much you lost with 2.5 coils removed.... Clipping another 1/4 coil when it was still shooting 735 may have been about right to put you at 700.... Also, since the first coil you removed was (or should have been) close wound (the end of the coil touching the next one), you only removed the wire diameter when you clipped the first coil off.... Taking another coil would remove the wire thickness, plus the spacing between the coils, which is a much greater distance....

Next time, if you cut off the first tight wound coil, just cut 1/4 coil or less after that and test each time you cut....Things can change in a hurry (as you found out).... BTW, the lack of efficiency may not be due to hammer bounce, but to having the valve still open when the pellet leaves the barrel (too much dwell).... caused by excessive hammer strike.... or a combination of that plus hammer bounce....

Bob

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PostPosted: Fri Jun 01, 2018 8:31 am 
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Shimmed the spring a little and ripped off 20 shots. It shoots and honest 755fps with 18's. I got 20 shots from 3000-2500 for an efficiency of about 1fpe/ci. Next time I tear the gun down I might reduce the shim slightly.


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PostPosted: Fri Jun 01, 2018 10:12 am 
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If you drop the velocity to just over 700 fps, you may increase your shot count by a 30-50%.... and the gun will be a lot quieter....

Bob

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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: Sat Jun 02, 2018 9:32 am 
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OK, we're getting there... Bob, if you didn't post all your crazy efficiency numbers I probably would have left it as is. Glad I didn't.

I reduced the shim slightly, it does sound quieter.

I also cut a new poly transfer port; I think my previous ports were cut too long and crushing in on themselves, restricting the air flow.

From 3100-1950psi in my 13ci tank, I get 56 shots averaging 770fps with JSB 18's. I could continue shooting for another 600psi or so, but as the pressure drops, my ES grows. From 3100-1950, I've got an ES of 2.5%.

This gives me an efficiency of 1.29 fpe/ci. Without building an RVA, I think I should be happy with this.


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PostPosted: Sat Jun 02, 2018 10:40 am 
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Good job that's sure a lot better than you started with. Bob's advice can always be taken to the bank =)

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PostPosted: Sat Jun 02, 2018 11:19 am 
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Thanks. My next step is to attach the bottle to the stock (it’s currently zip-tied in place with a remote fill line coiled around).

If you look up the qb78 synthetic, it’s got a small picatinny bipod rail that secured to the stock via small bolts and a backing plate. My plan is to grind out the picatinny rail so that it is more round and fits the contours of the bottle, then secure it to the bottle with a couple flat metal zip ties and some adhesive to keep it from spinning. Once I’ve got the bottle settled, i’ll Replace the remote fill line with braided line of the right length to tap directly into the end cap.


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PostPosted: Sat Jun 02, 2018 11:40 am 
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Sounds like you are planning to reverse the tank? Or is the braided line to run to the back, keeping the tank forward? If you do a reverse tank set up you can use steel fittings making everything more stable. Thats what I did, and cut out my wood stock to fit.

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PostPosted: Sat Jun 02, 2018 12:30 pm 
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Ya, reversed bottle. I did NPT fittings on a previous crosman build, and it is solid. Using a braided line and quick release, it makes it really easy to strip everything down to work on.

I've been eyeing those 4-12x44 scopes you're selling. Just about the right size for this build I think.


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PostPosted: Sat Jun 02, 2018 8:49 pm 
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Cool! I'd like to see some pics when you're done, that sounds like a nice setup. =)
If you wait til later tonight I'll renew the coupon on the canadian sales section for the forum members. 7% off with coupon code airgunforumfun
I won't get home til later tonight so it might be late but then i'll keep it permanent for anything on the canadian sales section.



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PostPosted: Sun Jun 03, 2018 12:58 am 
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Done, 7% is permanent now for members who use the coupon :)

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PostPosted: Mon Jun 11, 2018 7:18 pm 
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Well, as much fun as 25fpe was, I decided de-tune the airgun a bit. It was just way too much noise and power for my casual plinking. I also wanted to see if I could get the regulator a bit more consistent.

Switched some shims in the regulator and brought the set point down from 1400psi to about 1100. It now puts out a completely flat string from 3000-1100psi (aside from a 10fps regulator creep if I let it sit for a minute).

Power went from 770fps with 18’s to 695fps. I then reduced the shim in the hammer spring which brought the power down to 685fps. Efficiency doesn’t seem great, but i’ll run a full string before I tear it apart again and try reducing the hammer spring further.

I used metal pipe clamps to secure the bottle and bipod to the picatinny bipod rail on the stock. It’s really secure and didn’t require modifying the stock. I’ll try and post some pics tonight.


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