The only Gas anybody wants to be putting in their pre-charged rifles is BREATHING AIR, from an appropriate tested cylinder.
Forget all your crazy ideas of "you can get hold of this and that" or "my uncle joe makes his own methane,Will that do???"
You can pick up a cylinder in test for about 50 nicker, and it will cost you about £2 to fill each time with BREATHING AIR.
Edited to add, dont mean to rant, just read my post and i thought i sounded just like matt the bloody ratt on my high horse!! No offence mtr but you do go on sometimes!!