Sure an auto like the G18C will last way less longuer than a semi-auto.
But seriously, those are not the samething at all... An AEG SMG and a GBB pistol are two different things for different occasions.
For AEGs, you either have to buy a few batteries or wait. For gaz, you only option is to buy more gaz.
Start by getting age verified, and I'd suggest you get to a game and talk with people. This would be helpful without a doubt.