While looking for more pictures of the APS, I came across this section dealing with a type of ammo produced by the Russians which produced almost no sound. It was used for weapons that could not fit a silencer and was just as silent, if not more. Heres a webpage on it: http://world.guns.ru/ammo/sp-e.htm
I remember having read somewhere the US Army used something similar in Vietnam, mostly by Tunnel Rats. Calibres available were .38 and shotgun shells in 12ga. :-? I'll have to check my old books.
