OK Airsoft decided to swap their bulky HPA tank for a CO2 adapter kit on the MTW-308 to solve a rather awkward cheek weld situation. Apparently, the old tank was so wide that aiming required either a massive riser or crushing one's face into the stock, which isn't exactly the tactical look most players go for. The move to 12g CO2 cartridges provides about 100 shots per pop, which is plenty for someone who doesn't treat their airsoft gun like a garden hose.