Shorter barrels, lower pressures, and efficiency

[mackeral5]

This afternoon I ran a string through one of my QB78 based guns and based Lloyd's calculator it is running approximately 1.3 efficiency.  The gun has a 14" barrel, 13ci bottle, reg set at 1100psi.  Delrin poppet, 2mm valve stem, peek hammer, SSG, and .180 barrel port.  It was originally built for a 20fpe tune.  So roughly 810fps with CHP 14.3gr.  115 shots from 3000 down to 1000psi.  it will max out around 850fps. 

 With a 21.5" barrel this gun easily produces strings in the 1.5-1.7 range.  With a short barrel, what changes do you make to increase efficiency at the same/similar FPE?
 I already have a fairly quick opening/closing valve.  I could upgrade to a peek poppet, but I wouldn't think it will make that much of a difference over delrin at relatively low pressures of 1100psi.

What if I leaned on closing the poppet a little harder with a stronger return spring?  I have plenty of preload available so I can adjust HST to compensate. 

Is having too much plenum negatively affecting efficiency?  I could always stick a couple empty 12gr cartridges in the tube to take up space.   

Thanks in advance.

[rsterne]

A shorter barrel SHOULD decrease the efficiency.... You will be handicapping the gun's ability to make power by shortening the barrel, so the same amount of air will produce less FPE, reducing the efficiency.... That is the price you pay for less barrel length....

Anything you may be able to do to increase the efficiency with your shorter barrel would produce even better efficiency with the original longer barrel.... The only exception would be if you had the gun detuned to the point where the pellet is slowing down before it gets to the muzzle (very unlikely at a 20 FPE tune) with the longer barrel….

If you reduce the dwell of the valve, for example by fitting a stronger valve spring, you will reduce the velocity and FPE.... Compensating by adding additional hammer strike, on a regulated gun, will just get you back to where you were, but the gun will be harder to cock.... Unless your plenum is quite a bit over 1 cc per FPE (in your case larger than 20 cc), making it smaller will reduce the efficiency, because you will be reducing the average pressure during the shot....

Bob

[Gerard]

I'm using almost the same airgun, with a few differences in setup, resulting in very similar efficiency at 1.36. QB78D with 13.4" barrel, 13ci regulated tank with 1200psi output, hammer mass carved down to 38gr, internal SSG, spitting out 18.13gr JSB at 700fps for 130 shots from 3,000psi down to 900psi, the last half-dozen shots tapering down to about 688fps where I stop counting them into the string. I have a moderately strong spring on the delrin poppet with a shorter stem and a couple of O-rings on the block to stop the striker prematurely.

The same setup puts them out at an average of 785fps with a stock length barrel, for an efficiency of 1.71. But I prefer the rifle as a takedown which stuffs neatly into a backpack, so the stock barrel length is out for me, as much as I'd love that kind of efficiency. The slight edge I'm getting over your efficiency is likely coming from the SSG, and I suspect it could be even better if I went to such a light striker as yours. Might get around to making one sometime, as hey, who doesn't want even more shots? But the short barrel will always cost me some wasted air unless I were to incorporate one of those nifty balanced springless valves... Unfortunately that just looks like a bit too much work to figure out, with dozens of disassembly cycles and subtle tweaks before just maybe arriving at elite level efficiency. And nobody is offering a drop-in version for the QB78, so I'll just stick with my modified high-flow valve.

[Ribbonstone]

As usual...think Bob is right. Long barrels, even on effricient airguns, give more power...and becasue the same air per shot is used, give higher efficiency. May not give any longer shot count, but becassue the energy per shot is greater, the efficiency is greater.

Which really dosn't make a difference if what you really want/need is a short little stumpy carbine rather than  a long rifle.


Only occasioanly have I cranked a PCP/HPA rifle down low enough in speed/energy that long barrels didn't see an increase in speed/energy for the same air use.


HAven't had a lot of luck with a stronger return spring in most PCP's/HPA's, but occasionaly a stronger or weaker one helped me get where I wanted to be (am not one for open-ended goals, like "as much energy as possible").  It's not a one-way action, stronger spring also slows the opening, given a little less of a "sudden blast" of air; overall, it seems the effect is kind of self balancing.

[mackeral5]

I appreciate all of the feedback. 

I can't say that I am dissatisfied with the efficiency as it is, and I do understand the role the shorter barrel plays.  With a full barrel shroud over the 14" barrel this one is practically silent at 20fpe. 

But, we do like tinkering with these guns.  If a few more adjustments could yield more efficiency I would find that very satisfying. But I'm not losing any sleep about it.

It will be some time before I'm ready to go into this gun, but when I do I plan to at least try a stronger valve return spring.  Based on what that does or does not do I may play with reg pressure a bit.  I have a few bottles with varying reg settings. 

Gerard--I'm willing to bet I could see an increase of efficiency just by stepping up to 18.13's.  Originally I chose 14.3 CHP's for this gun as they are one of the less expensive, readily available,  plinking pellets.  I keep a few tins of 18.13's on hand for my Cothran tube, huma reg'd QB78.  it is a high 20-30fpe gun with 18.13's. 

The reason I keep questioning stronger return springs is my Edgun R3M standard....  It has an extremely heavy valve return spring.  For example, it is MUCH stronger than an OEM QB hammer spring--I would estimate at least  twice as strong.  I have to think that Edward chose such a heavy spring for a reason.  I've never tried a weaker spring on the Edgun so I don't know how it would respond.  But for some reason I think it will become less efficient with a weaker return spring.  I'll have to test this someday.....

Anyway I have enjoyed reading the information everyone shared.  Hopefully a few more will chime in.

[ChapoChoo]

My Gauntlet has a 9" barrel. My regulator is set at 1450 psi, I'm running the stock spring with one coil cut off and the hammer is lightened to 48 grams. I'm getting a pretty crisp shot, but definitely using more air than necessary with that heavy hammer. It's tuned to 12 fpe right now. My plan is to make an MDS hammer and try to get that shot cycle as quick as possible.

I haven't run a shot string, but based on sighting it in, I'm getting somewhere around 100 shots per fill. With an MDS hammer, I'm hoping to hit 150 shots per fill, but I don't know if a 1.3 fpe/cuin is possible with a 9" barrel. But having an 8" LDC that barely sticks past the bottle makes for a really small, quiet pest popper. I'm loving it!