I think it's more about the risk factor... Green gas works at low pressure.. Can't remember exactly, then you have Co2 which works at 800-900psi... However unlikely it is to have several things go wrong with the weapon at the same time, the risk of a bb with 800 psi behind it is too much for...
Don't bother with justbbguns... Both times I have ordered off them there have been problems.
You only have to do a Google search to find out just how many airsofters have been having problems.
STEER CLEAR OF JUSTBBGUNS.......You have been warned