Gas Blowback (GBB)

Green gas or CO2 stored in the magazine feeds a valve that opens on each trigger pull, sending a burst of pressure down the barrel behind the BB. On blowback models that same burst also drives the slide or bolt rearward and lets a return spring send it forward again, so the gun recocks itself the way the real firearm it replicates does.

That reciprocating action is the main reason people choose GBB over AEG: felt recoil, a slide that visibly cycles, and — on rifle-length GBBRs — a bolt catch and last-round hold-open. The cost is temperature sensitivity, since gas pressure drops in the cold, and gas magazines that need seals and lubrication to keep sealing.

New owners often over-fill a magazine hoping for more shots, which actually vents excess gas straight out the fill valve; filling to the point resistance stops, not further, gets the most out of a tank.