I use CO2 for two reasons:

1. Cost. I paid $20 for my 48 oz CO2 tank.
2. Number of shots per fill. I can shoot a whole case from one fill, and that's with an A-5.

How much would an HPA tank cost that could shoot a whole case? A lot more than $20 I'm sure...

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A5, 12" PPS Brass (.690) with Apex tip, Tacamo HK 416 shroud, Tacamo AK-47 Sight Cover, JCS Gold Powertube, E-Grip (APE), Core CQB Wire Stock w/ RVA, Palmer Inline Stabilizer, Lapco Cyclone Ratchet, 450 round Hopper, 48 oz CO2 with remote line