FPS... Joules... Ft/Lb... MPS..? All ways of measuring what's coming out of the end of your barrel, but only half of them describe your actual power...
FPS = Feet Per Second MPS = Metres Per Second, both measure the velocity of the BB, but that's only half the equation, without the BB weight these figures are pretty meaningless... Certain Airsoft distributors like to put very large numbers on their boxes and not mention that it's with a feather weight BB!
Joules & Ft/Lb (foot/pounds) both measure power, this is what your rifles are tested for when you're chronographed on site (most sites will check your weights, or chorno with their own BBs to check the power). The power is worked out by combining the speed of the BB out of the barrel and the weight of the BB.
In the UK most sites (in this area at least) run a 350FPS velocity limit (when measured with a 0.2g BB) this equates to just over 1.1 Joule. Everytime an FPS figure is talked about it should be linked to a BB weight, 350FPS with a 0.4g BB puts you over 2.2J and it to prison terms if you've got a full auto weapon...
If your rifle does 360FPS with a 0.2g BB it is over your sites limit, even if the FPS drops when your put a 0.25g BB you are still over the power limit, the power stays the same even though the BB speed drops. Using a heavier BB to get through a Chrono test is cheating the Chrono & you may be asked to leave the site or banned...
Power Creep... Now this is where it gets complicated... On some Airsoft weapons if you use a heavier BB you can get a higher power, the BB is in contact with the power source (the air or gas) for longer so gets given more power, this is much more pronounced on gas, some HPA rigged guns and some over volumed AEGs. For this reason the best way to chrono is with the ammo you intend to play with and work out what the power is with the BB weight you are using in game, it's more complicated, but is the best way to be sure exactly what power you are playing with.
Anyway, this was supposed to be a quick word!
See ya next time,
Ps, anything wrong, badly worded or just misspelled please let me know!