For shoulder bruising it is without a doubt the M1874 Sharps carbine reproduction loaded with hot 45-70 (405 at ~ 1900 fps). That little crescent steel buttplate tears you up in a hurry! The other "toy" that bruises the crap out of my shoulder is a Kalashikov (AKM) fired full auto for about 400 rounds in less than 30 minute (a welding glove is a recommended for the off hand).

Scott