i remember when these came out--i think they were fer the navy seals to ambush the trails in 'nam. got only 2/3 or 3/4 the velocity of a conventional round, but enough at close range to ruin somebody's day. of course, if the titanium nipple ruptured, it could just as noisy as the ones the rest uv us were shootin'.
mind yer topknot!
What I was briefed on years ago is: The casing is very strong, on firing the powder expands behind a plug that move forward propels the projectile forward out of the bore, the plug is caught at the end of the casing preventing any gas escape, hence no noise from the powder hitting the atmosphere, the projectile is subsonic so no sonic crack. Never got to use any, don't know if they are still in use.