Careful on the Gen 1 versions and a regulator (altros makes an in-bottle version), as the transfer tube/hole is relatively small: 3.7 to 4.0 mm for 25 and 22 cal, 2.5mm for 177. That makes quite a bit of pressure drop, since it is a long distance from the plenum to the barrel. This means you need a higher regulator pressure and you wind up with less shot count and lower fpe than you would like. The hole is limited by the barrel locking screws - if you open the hole past 4mm, you'll have to re-thread them with new screws.
I hope they enlarged this hole and the throttle knob, so the efficiency is better. The Gen 1's are a bargain at the moment, but I think I may wait for the Gen 2's as they offer quite a few advantages - larger shroud, picatinny rail top and bottom, regulator, AND higher power. The bottle versions should shoot all week without refilling...