Testing to reveal how pistol barrel lengths affect velocity *Posted as request

[Long_Gun_Dallas]

I'm thinking of building a compact PCP pistol with integrated moderator. But I think the barrel might end up being anywhere between 4-6 inches. I cant seem to find any data on anyone using barrels that short. Just wondering to what extreme power drop off would be.

*Updated Info*

Did some testing on my own, as I had a spare 700mm FX pellet liner.

Test mule was a 15" (380mm) FX Crown barrel. 16gr JSB hades baseline was 670fps at lowest setting 960 at highest.

7" average was 535fps low, 800fps high

6" average 515 low, 760 high

5" average 495 low, 700 high

4" average 460 low, 650 high



With that info, 5.5" seems a comfortable length to achieve target speeds of 550-600fps 

 

[jps2486]

Rule of thumb is that 1 inch of barrel length is 15 fps. You won't get much oomph from 4-6 inches.

 

[biohazardman]

I agree with the above 15fps per inch is a good number for co2 guns after the first 10 or 12 inches of barrel but if cut before that you will loose much more. You won't get much oomph from 4-6 inches is vastly understated. I would suggest a minimum of 8 to10inch or you will just be wasting a good portion of your air. Even 8 inches is a bit short. I try to keep my air pistols at 10-14 inches for best performance and air usage. This allows me to shoot the JSB 18gr .22 in the 600-700 fps range with excellent accuracy at 850-1200psi. Remember longer is better when it comes to airgun barrels especially when it comes to PCP.

 

[Long_Gun_Dallas]

Of course. I agree with every point that's been raised.

I have a 9" 25 caliber that shoots 20gr @750fps. Probably wasting a lot of air.

My 1322 with 14.5" shoots 650fps with 16gr



But I was really only looking for 550-600fps in 22 caliber. The idea was to make the gun as light and compact as possible, but still retain enough output to humanely hunt small game. Like the Ataman pistol, but with an integrated moderator.



I'm just trying to get a feel for whats feasible, and whether or not I should attempt the project.

I do have a spare 700mm pellet liner for my fx crown, I'm thinking of cutting down to get some figures.



So, let's say I could squeeze 800fps from a 9" 22. My 177 does 860, 25 does 750, so that seems reasonable. That means I'd only lose 75 fps going to a 5". I'd expect drop to be more extreme, but maybe not. I know throwing a regulator in there would really hamper output.

 

[biohazardman]

The drop off in fps per inch is not linear. Still I think that 500-600fps may be possible with a 6 inch barrel with higher pressures. Not sure on the 5 inch but even if you do manage somehow I would think that you will not get much in the way of accuracy with that much wasted airflow behind the pellet. That is of course unless you can strip all of that unwanted air off before the pellet gets to far. You know how that werqs. I wish you luck and have fun with your project.

 

[Long_Gun_Dallas]

Glad you mentioned that. Turbulence had crossed my mind as a potential issue. It wouldn't be too difficult to put a stripper before the internal moderator as the barrel would be shrouded. That would actually make the gun quieter. Utilizing more volume to disperse pressure/sound waves.

Thanks!

 

[Long_Gun_Dallas]

Updated original post 

 

[Vetmx]

When I decided to shorten my PP700, 8.5” was the magic number. Made them shorter and focused on my valve dwell the whole time. But 8.5” was where I wound up. No wasting air or diminishing returns. 

 

[Long_Gun_Dallas]

Got any pictures of the finished gun? 

And how, if you don't mind me asking, were you tuning dwell time? I tend to pay attention to how it sounds, and where peak efficiency falls. Always good to be backed off a bit from it. 

That one has a regulator, so power output should be significantly less than if it was unregulated. 

I figure I can get 550fps out of a 5.5" unregulated 22 cal without wasting a whole lot of air. Likely could do 625 or 650. 

 

[Vetmx]

Look at the top of page 3 in the Trail/pack small game topic in the pistol forum. That's my pistol. I wanted a more compact balanced pistol which the PP700 is not, to tote with me when I have to go investigate a downed critter or when I am just outside doing stuff. I wanted something that I wouldn't feel guilty banging around unlike my AP16. The valve on the gun has an adjustable spring. What a wonderful feature on such a cheap gun. A small compact gun is going to have a small air tank. A unregulated small air tank is worthless. Your shot string would look like a tire going flat. You never mentioned what kind of platform you are building this pistol around. If you want to hunt small game, you need accuracy. A tiny air tank under a 6" barrel unregulated wont have enough shots to have a bell curve. Especially since you are swinging for the fence velocity wise.

 

[Long_Gun_Dallas]

 

Ah, OK. Looks like you put a lot of time and work into that pp700. Can I ask how you went about shortening the air tube? I don't have the proper tools, so just figured I woukd have to get with a local machinist.

I saw you put a huma reg in there, too. I've got a 17 caliber pistol with one as well. It's tuned down to 630fps @110bar. Keeps shooting 630 when it falls off reg to about 90 bar. Still enough to take squirrel, but it's got a super high shot count and I deleted the reg plenum and hogged out the valve body. Not sure if that's a possibility with your gun, but might be a way to boost your shot count. Mine is 50. 

I'm looking at a 13xx or 22xx base. Going to be 100cc air capacity at 8" overall barrel length. (has to have space for moderator included) rough figures, anyhow. 

Here's a 5 shot string off my unregulated .25 pistol. 20 yards 625-670fps 25gr. It's not inaccurate. And I recon I could get more like 10 or better with shot placement like this in .22 the drop in POI after is very predictable on this 25. I'm fairly certain if the barrel was choked it would not be. 

 

[Vetmx]

I have my reg set at 130b and I shoot a 8.80gr pellet 700fps for 15 shots with an ES of 5fps. I left another 100fps on the table because shot count and ES went south. I’m glad I didn’t make it shorter because those 15 shots go quick when you find something fun to shoot at. My reg has no plenum. The plenum is in the frame of the gun. My Bandit or PP800 has an Altaros reg in it. In .177 it is shooting just like yours. Around 50 at just over 600fps. I got my extra velocity with a slightly longer LW choked barrel. Didn’t have to hog out the valve. Just shimmed the return spring to tighten things up a little. Keep us informed of your project.

 

[Long_Gun_Dallas]

Good deal

I did just score a cool scope for the pistol. When I said compact, I meant length-wise . I may end up not running a scope, not sure.

 

[Long_Gun_Dallas]

15.25" OAL, 7.6" barrel (not pictured), max 27fpe, over 100 shots at 11fpe, and dead silent.