Gas pistols come in either blowback or non-blowback format; they shoot at 300-400 FPS.  The blowback pistols have slides that kick back after each shot, delivering a recoil similar to that of when firing a real pistol.  Non-blowback pistols still utilize green gas to propel the bb but do not have moving slides that kick back.  All gas pistols are semi-automatic repeaters; some higher-end gas pistols feature full-automatic firing modes that shoot rapidly while the user holds down the trigger without letting go.