I think I see what you're getting at - in a specific gun the joules at the muzzle will be roughly similar for different weights of ammo.
The problem is that some people will lie about what weight they're firing. In your given example if someone says they're firing 0.2g and the chrono measures 340fps then the gun will pass chrono, if they're actually firing 0.28+ then they have effectively cheated the chrono and are in fact firing at MORE than the limit. The chrono cannot detect the weight so the operator has to go on trust that what you SAY your are using is what you are ACTUALLY using.
Having lived in Glasgow in the distant past, I am aware that it is the contents of Ian Bru cans that are made from girders. I am also aware that it tastes even worse than Vimto, which, given what the latter is an anagram of, is hard to believe.