Alyx, I'm pretty sure he's taking about propane, which is waaaay cheaper and less damaging to the gun internally. I've never even heard of a CO2 airsoft m4 ._.
Any tips you guys can give a beginning sniper?
get good at chambering in rounds quickly while still being able to maintain quick target acquisition and error compensation
Basically, be good and train for moving targets. Error compensation is always a bitch with changing conditions and moving targets
Also try to stay close or pay attention to the entirety of your team unless flanking or part of a squad, you don't want to end up being all alone with no backup on the opposite side of the field, or completely out of the way of action, because that wouldn't be any fun
Gas = C02
in airsoft 'gas' usually refers to green gas/propane
Also it's CO2 learn2chemistry