[L473ncy]

Duster (HFC 134A) or Propane ("Green Gas) are the most commonly used gasses.

Duster for low power (usually CQB and for less wear on the gun) and Propane for higher power or "kick" (should still get like 330 on a pistol though, enough for CQB here)

Oh yeah, also you're looking at minimum $500 for a (plastic) GBBR but more like $1000+ for a full metal GBBR.