More accurately called suppressors, the good ones will knock a good 30dB off the overall SPL - which means normal rifle rounds would still be fairly loud (between the loudness of the detonation along with the crack of the bullet breaking the sound barrier). However, if they're running subsonic rounds, there'd be very little sound.
It's the ammo, not the rifle. I can run subsonic ammo in any rifle I own, assuming it's the proper caliber.
Sonic rounds, a normal can pretty much takes the sonic "crack" away. 99% of the people wouldn't hear it in the din of noise in the stadium. Subsonic...out of a bolt gun...the guy standing next to it wouldn't hear it with the din of noise in bryant denny.
