my loudest would have to be my mini 14 believe it or not.i would think it was my mosin nagant.but when i shoot my mini without ear protection on my ears rattle after i shoot it.when i shoot my mosin this doesnt happen.
My H&R Handi Rifle in 500 S&W was my loudest gun until I sold it.
OUCH! From someone that lost a bit of their hearing over the years from industry and trade noise, this is something you will regret later in life. Hearing loss is no fun at all and really detracts from the quality of life.
For me, it would definitely be my Barrett 99 .50 BMG.