Why foot-pounds is the most meaningful airgun power rating
by B.B. Pelletier
I posted an article on foot-pounds back on July 15. A fellow calling himself "nordattack" took issue with what I said. I told him I would need to think about what he had said, then make my reply. Instead, I forgot to do anything about it. On December 21 an anonymous poster very clearly made the argument I should have made, so I'll give the credit for this post to whoever that was. Here is why I believe foot-pounds is the MOST ACCURATE method to rate power in an airgun.
nordattack proved my point!
nordattack said, "If we are simply told an airgun has 25 foot pounds at the muzzle, again without knowing the weight of the pellet, we are clueless. I mean it could be a 900 caliber pellet going 5 feet per second!" No sir, we are not clueless. I can tell you A LOT of things about an airgun that produces 25 foot-pounds at the muzzle. Here goes!
No manufacturer is going to show their own gun in a bad light.
We know that a spring gun almost always generates greater power with light pellets, while a pneumatic or CO2 gun does just the reverse. So, depending on the powerplant, we can at least determine the range of pellet weights used for testing, if not the exact pellet. If the gun in question is a Diana RWS 350 Magnum, which is a spring gun, and .177 caliber, it has to shoot 6.9-grain Hobby pellets at 1,277 f.p.s. to generate 25 foot-pounds. Since that's too fast for that rifle (it tops out at about 1,150 f.p.s. in .177), the stated energy is probably false. In .177, an RWS 350 Magnum probably produces a little more than 20 foot-pounds (6.9-grain pellet moving 1,150 f.p.s. at the muzzle).
If, however, the spring rifle happens to be a .22 caliber 350 Magnum, then to produce 25 foot-pounds it would have to shoot 11.9-grain .22-caliber Hobby pellets at 972 f.p.s., which IS believable! And, if it does that, it will probably also shoot 21-grain .22-caliber Kodiaks at about 700 f.p.s., which generates 22.85 foot-pounds. Can I rely on that number? Not exactly, but I can be reasonably certain that the rifle won't shoot .22 Kodiaks as fast as 775 f.p.s., but that it will be faster than 675 f.p.s. I arrived in that ballpark from the stated foot-pounds, even though I know that, to get those foot-pounds, the manufacturer had to shoot the lightest pellet it could find.
How do you know spring guns get more energy from light pellets, while pneumatics get more energy from heavy pellets?
Simple! I tested several different pellets and observed the results. You can do the same. While this phenomenon is not 100 percent guaranteed, it will turn out that way most of the time.
Knowing the muzzle energy tells you a lot about the airgun!
If you tell me a certain airgun produces 30 foot-pounds in .22 caliber, I know from experience that the gun has enough power to shoot 28-grain Eun Jin pellets. If it's a spring gun, I know it was tested with light pellets and will only generate about 25 foot-pounds with Eun Jins. Going to the energy calculator on the Pyramyd Air website, I can convert that to a muzzle velocity of 634 f.p.s. If the gun is a PCP, I know that it was probably tested with Eun Jins to get the stated 30 foot-pounds. That's 694 f.p.s. And, I know that it will get less energy with Kodiaks, perhaps 27 foot-pounds or so, which turns out to be 761 f.p.s. Those numbers are not exact, but they are in the ballpark, and that's what the energy level of airguns can tell me.
Velocity, alone, has much less meaning
Telling me the muzzle velocity alone is like saying, "Here's a partial score - Cleveland 11...." You didn't tell me who they were playing nor what kind of game they were playing. I remember reading about a Beeman P1 that some guy converted to shoot 1/8-inch ball bearings, just so he would get 800 f.p.s. The converted gun was a smoothbore, so the accuracy was lost, but he had that magic number! Velocity by itself is meaningless.
Muzzle energy tells the whole story!
Muzzle energy, however, is the combination of both velocity and the weight of the projectile. Because we know the general span of pellet weights for each caliber, muzzle energy clues us into BOTH pellet weight and velocity at the same time. Knowing the performance of the various powerplants (i.e., spring guns favor light pellets, PCPs and CO2 guns like heavy pellets) allows us to quickly determine the gun's performance range with all types of ammunition.