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I'm a former airsofter. You've been misinformed on the last bit and there's no need to give the "ban airsoft" crowd ammunition. :)

Airsoft rifles can't legally be over 328fps muzzle velocity with a .2g bb as this is 1 joule of imparted energy when something is hit, and at <1joule then the home office consider it a toy on the basis that it takes 1.3 joules to break the skin.

Anything over 1 joule makes it an air rifle, not a s1 firearm. Semi or auto makes no difference. This explains it usefully:-

http://www.mileoakshootingclub.co.uk/8.html

So by the formula on his site to get a airsoft rifle firing a .2gram plastic BB to >12 foot pounds you'd have to take the muzzle velocity from about 300 FPS to something like 1500FPS, which is something like mach 1.3. That's not even remotely acheivable from an airsoft rifle with a spring; the maximum acheivable is less than half that.