inonickname wrote:With hydrogen it exceeds the U.K. legal limit for air rifles..with a Q-tip..
Unlicensed air rifles to be specific. Appropriately licensed, there's no energy limit beyond what rifles you can actually buy in this country.
Also, to be honest, the sensible man's limit for unlicensed air rifles is 11 ft-lbf/15 joules. The merest fraction above 12 ft-lbf (in police tests, which aren't precisely defined) can cause one to be detained at her majesty's pleasure, so the wise leave a margin to avoid any possible problems.
As a closing point, I would say that my air rifle gets ~11 ft-lbf in an 9.5" barrel. You're getting ~14 ft-lbf in a barrel over 3 times the length and using a larger bore...
Impressive muzzle energy in its own right, but you're hardly comparing like with like.