Largest and most powerful shoulder fired guns I've shot were the 20mm Lahti AT rifle and 20mm Gustav recoilless rifle.

Biggest bang that I've actually pulled the trigger on was a 57mm anti-tank gun out at the Big Sandy Machine gun shoot in AZ.

Some of the boys and their toys:

http://www.break.com/userconte...mage-llc-216765.html

My favorite part starts around 1:15... 90mm AT gun.