
The working principle is similar to my pneumatic ballistic knife, a piston pushing on the projectile that is held in place with a ball detent. Moving the collar - either by pulling it back or by striking the whole thing against a target - displaces the balls and frees the projectile, allowing the air pressure to push it out.
Here it is in action against a defenseless biscuit tin and MDF sheets:
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More views, note that the fill valve is one of these as fitted to airsoft gas grenades and magazines. The penetrator is a length of 7.5mm dia HSS rod round to a point. Since my safety record is less than stellar, I've also added a grenade style safety pin



I've been using 850 psi of HPA from a paintball tank but it could also be used with one of these 12 gram fed devices.