Abstract: This invention relates to pneumatic guns that use a source of compressed gas to launch a projectile toward a target. A variety of trigger mechanisms have been used to apply the compressed gas to the base of the projectile in such a gun. In the present invention, replaceable discs of known rupture pressure threshold are mounted in the gas launcher between a compressed gas chamber and the launch tube. The gas launcher launches its projectile by the operator causing the pressure in the launcher to build up to the rupture threshold of the replaceable disc, at which point the disc ruptures and the compressed gas escapes from the compressed gas chamber past the rupture disc to push the projectile out of the launch tube. The rupture disc is a metal disc of known burst pressure.