This report covers:
- Good question
- Black Powder
- Compressed air
- How fast?
- Diminishing returns
- The deal with compressed air guns
- What about multi-pumps?
- And the single stroke pneumatic?
- CO2 guns
- Spring-piston powerplants
- Barrel length and accuracy
- Summary
Yesterday reader Yogi asked a question that inspired today’s report. He asked,
“Do co2 pistols/rifles gain/lose any power with a longer barrel? I know PCP’s gain power with the longer push. But co2’s????”
Good question
What a question. It’s time to examine the effects of barrel length on airgun performance. And I will start with firearms.
Black Powder
Up to the 1890s all gunpowder was what we call black powder today. It was so universal that the term black powder didn’t exist. It was just gunpowder.
Early in the development of gunpowder and guns people discovered that the size of the powder grains affected the burn rate of powder, and the burn rate affected the rise in pressure. Over many centuries we refined the grain size until today we have black powder in multiple grain sizes, with the common firearms sizes being Fg, FFg, FFFg, and FFFg. The more Fs the smaller the grains and the faster the powder burns. And something else — as fast as black powder burns, and when confined inside a barrel it burns at a rate of around 11,000 f.p.s., making it a low explosive, it still provides a push to the projectile far out into the barrel. In other words it is still burning and generating gas. So in black powder arms the longer the barrel is, the faster the projectile moves, within limits.
Compressed air
Now let’s talk about airguns. Precharged pneumatics (PCP) store compressed air until a valve releases some or all of it. Once the air pushes on the pellet it accelerates it down the barrel until the pellet exits the muzzle. Very much like black powder, the longer the barrel the higher the velocity for a given pellet in a given gun. The valve closes sooner or later and acts like the Fs in black powder. The faster it closes the sooner the pressure behind the pellet drops off, lowering, but not eliminating, the push on the pellet.
Think of an automobile you are trying to push. At first it moves very slowly, but the longer you push the faster it moves until it moves so fast that you can’t push it anymore. Compressed air works in a very similar way.
So, six guys push a car and it starts moving much sooner. Higher air pressure starts a pellet moving sooner — to a limit.
How fast?
Well, those six guys each run different speeds, so the longer they push the more guys drop out and quit pushing. Same for compressed air. At what point does a barrel get so long in a compressed air gun that no more velocity can be reached regardless of how much air pressure was behind the pellet? In my experience that limit is reached somewhere around 31 to 36 inches. In larger caliber barrels the air can push the longest. Ever wonder why the AirForce Texan barrel is 34 inches long? Now you know.
So — someone wants to build a .50-caliber big bore air carbine that gets 1,000 foot-pounds of muzzle energy yet has a barrel length of just 16 inches for ease of maneuverability in the woods — it ain’t a’ gonna happen. That short barrel robs you of 60 percent of your potential, or more!
Diminishing returns
Longer barrels do add velocity to a point, but after a certain length what they add isn’t that much. For a certain .177 caliber PCP an 18-inch barrel might get 950 f.p.s. with a certain valve tune (remember the F-size in black powder). Lengthening the barrel to 22 inches with the same valve might get the pellet up to 1,075 f.p.s. Stretch the barrel to 25 inches and the velocity might be 1,050 f.p.s. with the same valve and pellet. WHAT? A loss of velocity? But BB — you said a PCP maxes out at between 31 and 36 inches of barrel. Yeah, and I also said it depended on the caliber and the setup of the valve. Do you remember that? Everything works together guys.
The deal with compressed air guns
So here is the deal — the length of the barrel does affect the velocity, but so does the setup of the valve. So, for a given barrel length a valve can be adjusted to shoot a little faster or slower. Slower conserves air. Faster makes the muzzle blast louder. It’s a tradeoff, within limits.
What about multi-pumps?
Multi-pumps work the same way except that most of them dump all their air with the one shot. So for a multi-pump the amount of air you have at the start determines the velocity — to a point. Over-pump a multi-pump and the valve won’t be able to exhaust all the air. In some cases there may be enough air for a second shot, or, as we learned in the recent report, A short history of the multi-pump pneumatic airgun, guns like my antique Benjamin 700 may get many shots at reduced velocities.
And the single stroke pneumatic?
In the single stroke it’s a question of how much air can be compresssed in the one and only pump stroke. Because that is it. Barrel length maximums for an SSP are probably a small fraction of what they are for PCPs. And caliber? Forget it! The SSP is a .177 caliber powerplant almost exclusively.
CO2 guns
For CO2 guns we have a good body of data. Dennis Quackenbush made one of his XL rifles for me back in 1994 and I tested it at the full barrel length, then I cut off the barrel an inch at a time to see what affect it had on velocity. It was .22 caliber and here are the results. The barrel was taken from a Crosman 2200.
RWS Hobby
Bbl. length……….Vel
20…………………..644.8
19…………………..689.4
18…………………..680.0
17…………………..680.5
16…………………..670.6
15…………………..640.8
14…………………..638.6
13…………………..637.2
Beeman Kodiak
Bbl. length……….Vel
20…………………..548.2
19…………………..563.5
18…………………..561.9
17…………………..554.5
16…………………..546.5
15…………………..540.5
14…………………..518.2
13…………………..511.5
Change the valve tune and these numbers change. But they will never rise as high as those for compressed air because the atoms in air are smaller and move faster than the molecules in CO2.
Change the caliber and the numbers will also change. Generally a smaller caliber will shorten the length at which the barrel is most efficient, but again changing the valve can change that.
Can you see that between 19 and 17 inches the velocity difference are small, and as the barrel gets shorter the differences increase? Sure, more tests would refine our understanding, but this small set of data explains the relationship very well.
Spring-piston powerplants
Here the data were gathered and published in a book, The Airgun from Trigger to Target, by the father-son team of Cardews back in the 1970s. They discovered that the optimum barrel length for a spring-piston airgun was around 6 inches. Well, guys read that and started whacking off their barrels to get more velocity, only to “discover” that a spring-piston rifle that cocks with 35 pounds of effort with a 16-inch barrel takes 115 lbs with a 8-inch barrel! Duh! They must have missed that day in 8th grade science when levers were explained.
And there is a deal. The deal is, once the pellet accelerates to maximum velocity inside the barrel it COASTS the rest of the way! Yes, it does! The rifling has already engraved the sides of the pellet and, unless the barrel has tight spots ahead of where it achieves maximum velocity, the pellet is just coasting. Okay, so it looses 5-10 f.p.s. while inside the barrel. So what? Are you going to miss that? Nope. The guy who whacks off his barrel is the same guy who will tell you that he can’t afford a chronograph. He just reads stuff on the chat forums because THAT’S all true!
The TX200 Mark III has a barrel length of less than 10 inches. Yes the website says 13.19-inches but that’s on the outside. The rifled part on the inside is shorter. There are several baffles in front of it. You see — someone listened.
Barrel length and accuracy
Barrel length has NOTHING to do with accuracy! A longer barrel does not make a gun more accurate, despite what Bubba claims. Ten-meter target rifles have short barrels inside very long outer tubes. What increases accuracy is the distance between the front and rear sight. However. BB once demonstrated in Germany that he could hit a football-sized dirt clod (American football — think loaf of bread) at 50 yards with a Colt Detective Special snubnosed revolver, once he found the range and sight picture. He did it repeatedly to demonstrate it could be done.
Summary
There we are guys — BBs look at barrel length and how it affects performance in an airgun. Talk among yourselves!
Ah, here it is:
all I had to do, was, sign in, to find today’s interesting article about barrel lengths, hurrah! 🙂
Everyone,
Yep, we are up. I don’t know why WordPress acts this way and I’m sure not going to ask IT to do anything, or things could get worse.
BB
What’s the old saying? “let sleeping dogs lie”
rk,
Yep!
BB
“First, do no harm.”
And, speaking of barrel lengths…
https://www.nevingtonwarmuseum.com/v3-cannon.html
Hi Tom: an old blog of yours dealt with extreme gassing from the breech area of a Crosman 114. I’m having a similar problem with my 113 and was wondering if you ever found out how to fix this? Was it due to worn gaskets or O rings, a cracked exhaust valve, or some other problem? Thank you. You can send me email response: gene@etho.caltech.edu
geneak,
Extreme gassing at the breech means a leaky valve at the firing end. You need to rebuild that valve and replace all the seals.
BB
Thanks Tom. Did you do this for yourself or did you send it off to be repaired? I imagine that you would have to evacuate the stored co2 from the storage cylinder first for safety’s sake – what is a good way to do this quickly without having to dry fire it continuously until the pressure drops? Finally, where could I get the seals, gaskets and O rings to repair this classic?
geneak,
If it’s a fast leaker there should be no gas inside. One dry-fire is all it takes to know for sure. If there is still gas, try injecting ATF sealant in the next fill. The gun may not need resealing.
I sent m y off to this guy:
Rick Willnecker in PA. Contact him at http://www.airgunshop.net/ or email airgunshop@aol.com or call 717-382-1481.
BB
Did he say the problem was a cracked receiver and do you recall how much it was to fix/re-solder? My gun stores the co2 ok with no leaks. But when I fire it, I can feel gas escaping from the trigger and safety pin holes- enough so that I’m sure the lost pressure is affecting pellet velocity. If there are no suspect seals or gaskets, maybe the breech is cracked?
genak,
Okay, THAT is different. Some CO2 and pneumatic guns “leak” that way all the time. No cracked receiver. Just a matter of seals not stopping all the air. No leak.
If that is all that is happening you have no real leak.
BB
BB,
Interesting article and it got me thinking about something; is there a standard barrel length for the muzzle velocity claims on boxes of 22lr?
For example, it states 1070 FPS on the front of a box of CCI Standard Velocity, but I reckon it’s safe to assume that wasn’t from a snubnosed revolver barrel. Any idea what barrel length 22lr velocity tops out at?
Bob,
As I understand it the modern .22 LR barrel length is 19-20 inches, down from 24 inches in the 1950s.
BB
B.B.,
I find it interesting that so many things come into play when it comes to velocity. Some things just doesn’t make sense to me. An example is the Dan Wesson BB Revolver. The following barrel lengths with velocity stated beside it: 2.5″ is 344 fps, 4″ is 361 fps, 6″ is 426 fps and 8″ is 426 fps. I see as the barrel length increases so does velocity. But wait, why no increase between 6″ & 8″? Is it just wrong stated on the item. Or would if be the valve only allows so much C02. It couldn’t be due to 8″ being too long as we know that isn’t the optimal length for max velocity for a pistol (in general anyway). If I remember correctly, the same type of no gain was listed on the Colt Peacemaker 5.5″ and 7.5″ (When they made them, but they can still be had through PA through custom building). I don’t own a chronograph but I’m convinced the only way to be 100% sure is to test them with one.
Doc
BB
This is a fine tutorial that will make physicists cringe because folks can understand it.
Bet your pistol exhibition in Germany got the troops attention.
Deck
Deck,
There was only one witness to that. My buddy who thought it was impossible for a snubnosed handgun to hit a target that small that far away. I put two cylinders of ammo into that clod, once I found the range, which took about 3-4 shots.
BB
Crosman 2240 is a great little testbed for this. I found a chart which of course like an idiot, I did not save that showed roughly a 45fps increase when going from the stock barrel up to the 10.5″ and about the same fps increase when going from the 10.5″ to the 14.6″ and another 40 or so increase with the 18″ barrel where it then dropped back off a bit with the 24 inch barrel. Now that’s just the interwebs for you but it did correspond with my chrony testing.
I put the 14.6″ barrel on mine and gained 88fps over the short stock barrel.
I recall a multi-part guest series on customizing the 2240 posted here some time back. I believe different barrel lengths were part of the discussion along with some performance data.
Remarq,
Yes that was a long and really good series by Hiveseeker, over here /blog/2018/03/crosman-2400kt-co2-air-rifle-part-14/?swcfpc=1
We have not heard from Hiveseeker in some time, I hope all is good in his world.
Mike
Thanks B.B.,
That explains a lot!
-Y
PS guys at the range are drooling over the new FX 800mm barrel.
B.B.
So is the Cardews research valid for today’s springers? What was the standard average power then? Do you think that a 12-20 fpe modern springer can reach its maximum potential in such a short barrel length of 6 to 8 inches? No need to mention the leverage factor again.
Bill,
I think it’s still valid and the TX200 barrel proves it. It’s less than 10 inches yet the TX hits above 1,000 f.p.s. in .177.
BB
The Cardew stuff was right with what they had – a leather-seal, slightly diesellling HW35, though I think it was 8” not 6”. I think the more modern view is that 12-14” is the best minimum, depending on a host of other factors, and that, as BB indicated, longer isn’t worse. ,
“…BB once demonstrated in Germany that he could hit a football-sized dirt clod (American football — think loaf of bread) at 50 yards with a Colt Detective Special snubnosed revolver, once he found the range and sight picture.”
Great stuff! Elmer Keith’d be proud of you. 😉
When I was a young journeyman industrial maintenance man I built an air powered long gun to kill pigeons that had infiltrated our factory one winter. I and my fellow maintenance men did it for sport during our lunch break and everyone but me used Wrist Rocket style slingshots to do the deed.
My barrel was a phenolic resin and linen tube that was 30″ long 1/2″ OD and 1/4″ ID and was powered by an air over water fire extinguisher tank, sans water and filled to 125 PSI air pressure, ’cause that’s all we had available at our plant. It fired a 3/8″ long .250 steel dowel pin and the “trigger” was a blow off nozzle like many of you probably have on your shop compressors to blow dust and chips away.. I would simply squeeze and release as fast as I could. I don’t know the weight of the dowels or the resulting velocity and energy that was created, but I know that it punched a bunch of holes in the steel roof of our factory building and that roof was at least 35 feet over our heads from where we were shooting. That was after going through a pigeon. I got in a bit of trouble over it and had to stop. I changed over to shooting rats around our 3 trash compactors outside the building (I worked the night shift all my life, We got bored sometimes.)
This same gun would shoot a 1/8″ 6011 welding rod all the way through a corrugated steel wall at about 20 feet.
The point of my story is that a lot of energy can be generated with a relatively low air pressure, if the barrel is long enough and if you keep that air supplied for long enough. Remember that I was relying on my reflexes to open and close the air valve and even at the tender age of 23 I wasn’t as fast as a knock open valve, which means I applied a lot of air, at low pressure, to my “gun”.
Half
BB,
What is it about the tube in bolt arrangement of a gun like the original AirJavelin that allows it to propel a 170 grain projectile (air bolt) at 300 fps for an energy level of 34 fpe at the muzzle? The bolt is about 15″ long and one can presume that the “barrel” tube is of a similar length, so 30 inches, all in, if you think of the barrel as sort of accordioning out of itself as the bolt slides down the tube. The CO2 is only about 900 psi, so how come so much speed and energy? Would a 30″ inch barrel of the same diameter (let’s assume the average between the ID of the tube and the ID of the bolt) firing a more conventional pellet/slug, of the same weight, with the same valve and CO2 source, be expected to match the energy level? If not, why would that be?
Half
Half,
Huh?
BB
BB,
Firstly, I guess I need to establish that you think an air bolt over a tube acts like a barrel that is twice the length of the bolt, as the volume in the two tubes increases as the bolt/arrow slides forward. If you agree with that, then I ask if a regular barrel of that diameter would propel a conventional pellet of the same weight with the same energy? If you don’t think that it will, do you think there is something special about the tube over tube arrangement in an air powered arrow launcher that allows it?
I’m trying to understand how so much energy is produced with CO2 and whether it would be possible to get that energy with a more common barrel and bullet setup. I’m also curious about why the bolt slips over the tube in one launcher and inside a barrel on other launchers. It seems that the over a tube method would require so much gas just to fill the tube before it even started acting on the arrow, but on the other hand, the arrow that is fitted over a tube would have less friction, possibly, to deal with. Just wanted to know if you could shed some light.
Half
Half,
I need to think about this but right now no — I don’t think the tube doubles the barrel length.
Second I doubt you can even find a pellet that weighs what an arrow weighs, so no joy there.
I think you need to drop some Mentos into Coke!
I seriously do not know what you are asking. Airgun arrows move very slow compared to pellets — 400-600 f.p.s. max. So low pressure gas can push that fast. We aren’t talking 1,000 f.p.s.
BB
B.B.,
You should have been my higschool Calculus teacher. Once you mentioned the image of men pushing a car, the entire article made sense.
I once asked grandmother why were bees yellow? She replied….. “to distinguish the bees from flies. Those are black.” I wanted to ask why flies were black? Somehow, I knew not to .
Alex2no,
I wish I could claim that I did that to help everybody, but it was just to help BB. Isn’t calculus what shifty-eyed Latins do before striking a deal? 🙂
BB
Thought calculus – or calculi – were what cross-eyed ancient Latins loaded their catapults with when besieging fortifications. This must sound like something right out of the Pun-ic Wars, B.B.
Regarding springers – it is wise to cut the barrel to the “optimum minimum”. Because of the recoil and powerplant vibration it is good to shorten the “pellet in barrel” time. This optimum minimum is the lenght where the pellet is accelerating. Springers are not PCP. Usually 12inch barrel lenght is more then enough.
Tomek,
But one must also keep in mind that the effort to cock a breakbarrel rifle goes up the shorter the barrel is. As in all things there is a compromise, hence barrels are sleeved to be longer than they actually are to provide a longer lever.
Siraniko
Siraniko,
Exactly, sleeved or with some modulator will do the job. It all depends on the powerplant, how kicky it is. I have .22 brakebarrel with 18.5 inch long barrel and accurate as it should be, underpowered a little though.