A compressed gas powered gun having a valve assembly (40) for regulating the pressure from shot to shot, a trigger assembly (36) which alters the mode of operation of the gun and a bolt (38) for providing a recoil action and for releasing the gas from the valve assembly (40). The valve assembly (40) provides sufficient pressure to expel a projectile and to re-set the bolt (38) into its firing position. The trigger assembly is formed from a trigger (26), a four-position selector switch (46), and a sear (74). The gun uses an operation rod (118), which is integrally connected to the bolt (38) to advance the gun's magazine.