When dealing with Supersonic bullets (.223/etc), you are basically putting a muffler on a V8...
it'll be a little quieter and can modify the sound, but you'll know when it goes off
You do get the sound of the bullet breaking the sound barrier as it goes down range, but you do loose 95% + of the muzzle blast. Its not much louder than an air rifle or .22 in the open, and identifying where the shot came from is difficult, as there is no muzzle blast.
I have a AAC M4-2000 on my one AR, and I can shoot it from inside my carport with standard ammo, and it doesnt even set my ears to ringing. If I shoot a .22 or my .17 from the same spot, I cant hear right for days.
I know this is posted the bolt section, but this has virtually no recoil, and is easy on the ears.