How barrel length affects airgun performance
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
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????”
What a question. It’s time to examine the effects of barrel length on airgun performance. And I will start with firearms.
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.
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.
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!
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.
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.
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.
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.
There we are guys — BBs look at barrel length and how it affects performance in an airgun. Talk among yourselves!