Mike Morton explains how to calculate your PCP’s power curve sweet spot using a chronograph
Q. I recently purchased an AirForceOne chronograph. I own a .22 rifle which I thought was a bit low on power. The seven shots I fired through the chronograph read 495, 499, 503, 503, 503, 507 and 503 feet per second. But converting the FPS to muzzle energy without knowing the mathematical formula has me at a loss. Can you help?
A. Chronographs are great not only for ensuring you’re staying within the law, but also for calculating the sweet spot on a PCP’s power curve. They can also be used as a diagnostic tool if, as in your case, you suspect your rifle is not performing quite as it should.
To calculate the power of your rifle you’ll need to know not only the average muzzle velocity, but also the weight in grains of the pellet you’ve used. I usually chrono 10 shots as it’s easier to divide by 10 to get the average. But the figures you’ve supplied for seven shots show your airgun has an average muzzle velocity of 502 feet per second and a spread of 12 feet per second. If your rifle has a regulator, it’s always a good idea to fire at least one shot before putting the rifle over the chronograph to ensure that the regulator is filled at its correct operating pressure.
One thing you didn’t mention, which is vital to working out muzzle energy, is the weight of the pellet you were using. This will have a huge effect on the result. If, for example, you were using a .22 pellet weighing 16 grains, then your rifle is putting out just 8.96 foot-pounds. But if you were shooting a heavy 20 grain pellet, your muzzle velocity would now be a much healthier 11.19 foot-pounds.
The formula to calculate muzzle energy is: pellet weight x muzzle velocity x muzzle velocity, all divided by 450240. If you don’t want to do the maths yourself, you can download a free airgun and ballistics program from Hawke called ChairGun (www.hawkeoptics.com/chairgun). This sophisticated program has many features which we don’t have room to go into here, but you can simply input your pellet weight and chrono data to find your muzzle energy with a simple click.
In your case, you have a third option. While the AirForceOne chrono doesn’t enable you to input pellet weight, you can download some tailor-made software to do just that and work out your muzzle energy.