So far, my 300 Winchester magnum Remington Sendero and my S&W 500 Magnum 8" barrel seem to be my loudest.
Course the 338 Lapua Magnum Remington 700 Police that I just bought and am in the process of building will likely be louder when I get around to shooting it.
CAI CETME Sporter with the early brake which should have been called a Blast Intensifier. No rifle I have ever fired including Belt Fed Machine Guns have even come close except for the M2 .50 BMG.
Well, they're no .50BMG, but I've shot my brother's S&W 460 Magnum and 500 Magnum revolvers. Every time we shoot them at the indoor range, all the other lanes STOP, step back, and say 'WTF!!!' These things blow the paint off the lane partitions! Loudest thing I've ever shot.
The loudest firearm I ever heard was a nut at the range who had a custom T/C Contender pistol in 300 Magnum. Not only loud but the concussion wave went through your body from 20' away. It was so uncomfortable, everybody else gave up shooting and left him alone.
I've never heard it, but we were told in training the loudest sound in a gunfight is a "click" when you expected a "bang." I believe it holds true for any gun!