Note: Check that your Chronograph is accurate especially if it is one of the cheap types. A good chronograph should be near a foot long for accurate speed measurements.
To check the power of your gun you need a good Chronograph which will measure the speed of the pellet or projectile, when fired through it.
When you know the speed of the pellet then you can work out the energy. To work out the energy you must know the weight of the pellet in grains. The method of working out the energy is:
Is your gun doing the legal limit?
The reason some shooters think they need more than the legal UK limit of 12 ft. lb. is probably because the gun they think is just under the legal limit is sometimes a lot less. Most of my customers believe that the new gun they have just purchased is just under the legal limit. It is usually a big surprise when they find that the gun is only doing between 8 ft. lb. and 10 ft. lb. We have found that some guns are only producing 6. The very worst we have tested was a very well known Pre Charge air rifle that was only doing 3 ft. lb.
I only believe the power output when I see it myself. And I can check the pellet weight claimed, and see the velocity reading. I have put some pellet weights on this site.
When you see the velocity yourself, and know the pellet weight, then you can work out the energy.
To work out the power of your gun, the formula is:
V2 X PW / 450240
This just means that you times the velocity by itself `(say velocity is 610 fps X 610 fps = 372100 ) then X (times) the pellet weight (372100 X 14.3 grains = 5321030) and then divide by 450240 (5321030 divided by 450240 = 11.8 ft lb.). This means that if the gun fires a 14.3 grain pellet at 610 feet per second then the power is 11.8 ft. lb. This is a good power to have and should still have enough energy at 40 yards to kill a rabbit. This is of course assuming that you can put all your pellets inside a small circle at that range.
If you have a firearms certificate then the maximum power we recommend is 25 ft. lb. this is because we have found the best accuracy is found at or under this level. This power will still humanely despatch rabbits without the danger of killing some one within a mile.
The Maximum power I normally recommend for FAC is 25 ft. lb. The reason I say this is because of the pellets, most pellets cannot take much more than this without getting distorted. When they get distorted to any degree then the accuracy gets worse.
You can use heavier pellets to compensate, but the velocity will be less and you end up back where you started with a poor trajectory. There is no reason to go down the heavy pellet route, because when you have about 25 ft. lb. then you still have enough energy at 90 to 100 yards to humanely kill rabbits. So the limiting factor is accuracy.
You should not even consider shooting anything larger than rabbits with an FAC air rifle, this would be cruel as you cannot ensure a humane kill.
If you have a Firearms Certificate for an Air Rifle, then we should be able to help you. If you don’t then please do not ask us to tune your gun over our limit. the answer is NO, even if you were my brother.
In stock at the moment we have a Second hand Webley Patriot in .22. This is only £115.00 and a Sheridan .20 £120.00 if you have a FAC for that calibre.