Abstract: A mechanism for a gas powered gun where a tubular bolt 10 of a paintball gun communicates with a cylinder 16 through bores 17 when in a forward, firing position, allowing compressed gas from the cylinder to expel a paintball. A port 20 whereby the cylinder communicates with a source of compressed gas is occluded by the bolt 10 except when a circumferential recess 23 in the bolt is in register with the port 20. By this arrangement only gas stored in the cylinder 16 is used to fire a paintball and the source of compressed gas is not in communication with the cylinder except when the trigger is released and the bolt 10 is withdrawn.