In real life, I would have to say the Mosin Nagant 44 carbine, with heavy ball ammunition. It's a slow thud that hurts even with ear protection on, and the fact that the clothing of anyone within 20 feet around waves from the burst is impressive. I've never found a firearm sound in a videogame that really felt realistic, which is probably good, because even firing a .22 without ear protection makes your ears hurt. Firing a .357 in a 12x12 room without ear protection will permanently damage your hearing, and it just sounds like a loud bass bang followed by ringing the rest of your life.