Socom 16 in .308 or savage axis in same caliber. Both rock hard.
930 Mossberg running 3" nitro steel magnums @ 1750 FPS makes a really nice Boom, Boom, Boom.
probably not loudest here by far, but my Colt Commando clone with its 10.5 bbl and 5.5 flash hider gets everyones attention. calling it a flash hider tho is like calling glasspacks a muffler..all it does is make it louder and throws more fire.
short of big bore guns its the loudest thing ive fired.