by B.B. Pelletier

We received a lot of comments to last Friday’s post, Why foot-pounds is the most meaningful airgun power rating. One of them was a question from “cold shooter” about something I said in that post: “Please explain why a CO2 pistol would prefer or gain more energy from a heavy weight pellet.” The full statement I made went something like this: “Spring-air guns are more efficient (have more power) with lightweight pellets, while pneumatics and CO2 guns do better with heavy pellets.” Today, we’ll examine this phenomenon.

First, a new book
Trust me, this book is very much related to this discussion. The Practical Guide to Man-Powered Bullets by Richard Middleton has just been published. It’s an excellent discussion of energy transfer, momentum and the design of catapults, crossbows, bullet-bows and airguns. I think this book clarifies the spring-air versus pneumatic question quite well. It’s related to energy transfer.

If you want to understand airguns better, this new book provides many excellent discussions on energy, momentum and the ballistics of airguns.

To acceleration, time is everything!
One of our readers was the first to explain this in the comments to the same posting where the question was asked. He said, “A quick answer to cold shooter’s question. Heavier pellets stay in the barrel longer, which allows the released CO2 gas more time to expand and transfer more thermal energy into kinetic energy.” To simplify that a bit, the more time the gas has to push on the pellet, the faster it will go!

Spring-air guns are quick!
A modern spring-piston airgun, like the Beeman R11 MkII generates power when a steel piston is rapidly shoved forward by a coiled steel spring. The piston compresses the air in front of it, shoving it through a tunnel called a transfer port, where it travels to the breech of the barrel. If a pellet is in the breech, the air is stopped and cannot move forward, so the air pressure builds instead. An immovable pellet on one end and a piston compressing air on the other causes the air pressure to rise quickly in the transfer port. When the pressure reaches a certain point, the tiny lead pellet can no longer restrain it, so the air shoves the pellet up the barrel.

Springers just puff!
While the air compressed by the piston is at high pressure, there isn’t very much of it. Once the pellet starts moving, the pressure starts dropping as the volume of the bore behind it increases. By the time the pellet has travelled 9″ to 11″ up the bore, the air is almost back to normal pressure, so it stops shoving the pellet. By this time, the pellet is traveling as fast as it will ever go. Since the time of acceleration is very short, lightweight pellets tend to go much faster than heavyweights. They resist the air pressure less so they start moving sooner, affording more time to accelerate.

While gas guns blast!
Both pneumatics and CO2 guns use a greater volume of gas than the spring gun generates. In the case of CO2, it’s not at a very high pressure (900 psi, compared to about 2,000 psi in a spring gun) but there is so much more of it that it keeps on pushing far longer than the tiny puff from the spring gun. As long as the barrel is long enough to put the pressure to good use, both pneumatics and CO2 guns will accelerate heavy pellets to higher velocities than spring guns can, and that’s where they get their extra power. If you were to cut the barrel of a pneumatic very short, you would also cut the power. The AirForce Talon SS provides an excellent example of this. With the standard 12″ barrel, the gun gets 830-850 f.p.s. with .22 caliber Crosman Premiers. When you install an optional 24″ barrel on the gun, the velocity of that pellet jumps to just over 1,000 f.p.s. – without changing anything else!

However, the Talon SS still generates more power with its 12″ barrel with heavy pellets than it does with light ones (26 foot-pounds with Beeman Kodiaks versus 23 with Crosman Premiers), because the pellet is accelerated all the way to the end of the barrel.

CO2 is even more dramatic!
CO2 is a gas that changes pressure with temperature, so if the gun you shoot is relatively warm, the gas maintains its pressure much longer. Also, the large size of the CO2 molecule means the valve has to remain open longer, so fresh gas is replenishing the supply in the barrel. The results can be dramatic! A Farco air shotgun, for example, can generate 100 foot-pounds on a warm day, due to a very long barrel and CO2. And, the Farco provides the perfect example of light versus heavy pellets. A 120-grain .433 ball produced 65 foot-pounds in my gun, while a 245-grain load of shot made 105 foot-pounds on the same day. It doesn’t get more dramatic than that!

This is a good experiment for your new chronograph. You will find some anomalies, but in general, this rule will hold true.