Originally Posted by Brian.Samson
grain isn't a force, but that's not the problem.
The problem is that Foot Pounds Force (FPE) which is how the UK law defines a legal airgun, is a force that is different depending on the value you use for gravity (which believe it or not, isn't constant)
Joules would be a much better definition in law, but we have a crazy law
Actually, grain is
a force... weight is... F= ma = weight. Grain is a weight measurement. Weight = force. Essentially scales measure the force a mass exerts on them.
We convert grains to pounds (the 7000 x gravity x 2) bit which is the constant used... except that because various people keep sneaking in their own measurement of gravity, the constant changes, which changes the output.
As Rich alludes to, UK firearms legislation says 12 ft-lb is the limit, before we need a firearms certificate, but doesn't actually state how it's calculated. Everyone has run around making assumptions, but unless anyone has any detailed case history, I'm not sure it's been defined yet in court.
I bet Alex uses something sensible like Joules, which uses mass and not weight/force and therefore doesn't deal with gravity at all, because as we all know mass is a measurement of substance and doesn't change no matter where you are in the universe (well, when stationary, but we'll leave Einstein out of this one)
Joules is a scientific measurement of kinetic energy. Ft-lb when calculated by the normal accepted equation, is really just a bit of a fudge which varies a bit.
If we really want to be pedantic we should check that the pellet's weight in grams is not a weight and it's mass. But I think measuring their volume and density might be pushing it a bit and I don't fancy using a balance in the field to check something that weighs around half a gram.
I just wanted to be sure that when I do a simple list of pellet weights and competition legal speeds for the side of the chrono for reference we weren't going to get someone saying they were wrong because chairgun uses something else.
And I was bored.