http://world.guns.ru/ammunition/russian-special-cartridges-e.html

The SP-3, SP-4, and PZAM cartridges are supposed to be "silent" cartridges...would they be legal?

"Silent" loads are easy enough to load up. Usually its a fairly small pistol powder load for a carbine or rifle bullet.

The idea is that the gases use the length of the barrel to slow down to below supersonic by the time the bullet exits the barrel. The results aren't "silent" but they don't have the load report we associate with most shooting.

"Silent" loads are also ~very~ low velocity. There is a load for 44 spec/mag that is about 650fps. 44 mags are usually around 1200fps or higher, much higher from a carbine. So there isn't a whole lot of power in "silent" loads. These are "bunny fart" loads.

They are still potent enough to do tons of damage, though. A "silent" 44 mag load still has more kinetic energy at 25 yards than a 22lr at the muzzle.

I don't think they are illegal since it is essentially crippled ammunition. Some crafty law makers would make it illegal in a snap if it were ever used to commit a crime, though.

Oh, they also don't work too well in semi-autos unless you change the springs and/or gas system. Not nearly as much force being generated as with normal loads.

I use silent /22 loads for indoor plinking.  Colibri / Super Colibri from Aguila make less noise than a spring-driven air rifle, but have considerably more umph.  And the CCI CB series is quiet enough to shoot safely without hearing protection.  Not the same league as what you're talking about, but much closer to actually being silent!

Those are different as the piston inside the brass seals off a lot of the blast.  They're definitely going to be moving a lot slower.