The loudest gun i have ever shot is a 460 S&W out of a Tc Encore pistol Not only was it loud but i could feel my close move and see all the leaves on the ground move every time i shot it.It was a canon.
I have a Russian M44 Mossin Nagant. It's a 7.62 x 54R and will put out a foot and a half muzzle flash in broad daylight. I'm not allowed to shoot that at my in-law's house because it vibrates the pictures on the walls with each shoot!
I had a rangemaster a few weekends ago come over to me while I was shooting my Rem 700 sps .308. He picked up a spent cartridge of mine and said " A .308? I figured you were shooting something bigger, that darn thing is loud!" it's a factory gun, maybe it was echoing off the walls at the range.
I don't own it but my wife's brother-in-law's 22-250 really surprised me. His desert eagle is also pretty loud but I think that it is so close to my head (my arms aren't that long...)