I have many pellet guns, some of which do exhibit a supersonic crack when fired. Most are .22, though and getting 950-1000 FPS with a 22 pellet rifle is pretty strong. Also, have 9mm air rifle that will put a 60g pellet through a 2X4. Have a .308 air rifle that shoots cast 30 carbine bullets.
My 22 cal suppressor is from the lower end of the price spectrum but still tames the supersonic crack from bulk ammo. I have a chrony, a real time audio spectrum analyzer, and a few dB meters. I have not, however, set all the equipment up and made measurements. I am just speaking from experience. I've threaded many 10/22 barrels, many other AR style 22 semi auto rifles, and a bunch of 22 pistols. The short barrels on the pistols usually keep the high velocity ammo from supersonic velocities. The rifles and carbines experience more suppression than you would think, the supersonic crack is not there.
My point is before you go taking measures to lower velocity in a rifle or carbine you should try the suppressor and see where it can get you. If you want quiet pest control there are air rifles, some with suppressors that are called "moderators" or "lead dust collectors" or "noise dampers" that are very effective at short ranges and can approach 22LR capabilities.
As for the 308 vodoo magic, my buddy has a can that quiets standard 308 hunting ammo way more than you would think it could in such a small package. He's a dealer/manufacturer and has had many suppressors.