How barrel length affects velocity in a pneumatic airgun
by B.B. Pelletier
How barrel length affects velocity in a CO2 gun
I didn't intend for this to become a series, but the comments to the first post about CO2 guns requested this post. Okay, I offered it at the end of that first post, so I guess I wanted to do this one, as well.
I was really surprised by the results of that CO2 barrel length report - they weren't what I expected at all. When I thought about it, the reason I said that I thought a 24" barrel on the Crosman 2250 would increase velocity by 100 f.p.s. is because most of my experience has been with air, where there really is a dramatic difference like that! Today, I will make the case that increasing barrel length in a pneumatic gun readily increases velocity.
Airguns of the past had longer barrels!
My first piece of evidence comes from the past. All the big bore airguns of old had long barrels, and the reason they did was for velocity. Those old guns (and I am including the few rifles among the guns) operated on air pressures of 500 to perhaps as much as 900 psi, though I suspect that last pressure to be a bit high. Most of them were comfortable with 600-700 psi. We know this for a fact because Dennis Quackenbush and Tom Gaylord did a study of vintage pump designs and determined their limits. They demonstrated that a single-stage hand pump can generate up to 1,275 psi if the person doing the pumping is heavy enough and the pump piston is small enough. They also found a practical limit for the old pumps, which actually delivered around 800 psi, give or take. Their study is published in Airgun Revue 4.
The ONLY way to generate higher velocity with pressures that low is with a longer barrel. Indeed, when we measure the length of vintage big bore airguns, they have barrels that range in lengths from 28 to 33 inches. There are guns with longer barrels, but there aren't many with shorter barrels (except for handguns and guns made for children). The locks on the old guns held the valves open much longer than contemporary airgun valves. An examination of the vintage valves reveals huge air passageways! Everything was designed to move large volumes of air. It takes a long barrel to take advantage of that.
The long barrel on this Shembor repeating air rifle is typical of all vintage big bores.
If you're interested in how these old guns operate, check out the DVD Antique Bigbore Airguns - From Novelty to Necessity.
A more elegant proof
Proof No. 2 is a remarkable air rifle that has stunned the world! The USFT rifle produced by Mac-1 Airgun delivers 55 shots of a 10.6-grain Beeman Kodiak pellet traveling at 900 f.p.s., and it does it on a starting air pressure of just 1600 psi! When the string is completed, the gun will still have 1100-1200 psi remaining, so all that work is done on as little as 400 psi of air. The air reservoir of the rifle is HUGE! Although the air PRESSURE is relatively low, the VOLUME is very great. Obviously, when a given volume of air is made smaller without air loss, the pressure increases. The USFT rifle reverses what most modern PCPs do. It uses a larger reservoir to lower the pressure of the air. Same amount of air - bigger volume.
USFT rifle from Mac-1 Airgun does more with less air pressure than any modern air rifle.
The ONLY way to use this lower-pressure air effectively is with a longer barrel. When the pellet leaves the barrel, the push provided by the air stops. The USFT rifle uses a 25" Weihrauch barrel, which is the longest one they make. If they offered a 28" barrel, I have no doubt it would be incorporated into the USFT - not to increase the velocity, but to lower the air pressure needed to achieve 900 f.p.s. By the way, this rifle starts at $1,650 and goes up rapidly past $2,000 (and Mac-1 is backordered for several months from the demand). Top competitors are switching over to this purpose-built field target rifle, and they all agree that 900 f.p.s. is the ideal velocity, when accuracy is on the line. This gun just took four of the top five places at the U.S. Nationals, including first place.
The final proof
The final proof is so positive that there is no way to refute it. The .22 caliber AirForce Talon SS gets 820-850 f.p.s. muzzle velocity with a Crosman Premier pellet. It has a 12" Lothar Walther barrel. When you substitute a 24" Lothar Walther .22 caliber barrel, that SAME GUN jumps to 1,000 f.p.s or a little more. That's pretty amazing, but it doesn't end there. Because the Premier is going too fast for good accuracy in the longer barrel, loading a 20.5-grain Beeman Kodiak will slow down the speed to 920-950 f.p.s., but the ENERGY increases from about 32 to over 40 foot-pounds. And, a 28-grain Eun Jin will take the energy up to 45 foot-pounds with a velocity of 850 f.p.s.! The basic rifle with its shorter barrel was getting about 23 foot pounds from the Premier!
You cannot keep extending the barrel forever. There is a turnaround at some point, and it will relate to the caliber of the pellet. A smaller pellet equals a shorter optimum barrel length. But air being so much thinner than CO2, this turnaround point will be much farther from the breech than what we saw in the CO2 test.
To the best of my knowledge, this has never been thoroughly tested. If it has, I don't know where to find the results.