Biggest shoulder weapon for me is either the .470 Nitro (one round from a Bud Searcy double rifle), or the .50 BMG in a couple of single shots and one .50 MG. The .50 single shots were more comfortable to shoot than the .470. The Big 50 was more fun.

Largest handgun a Desert Eagle in .50 AE (I think). It is owned by the same guy with the .470. A .45-70 revolver he owns was fun to shoot too.

Another buddy has a black powder cannon I have gotten to shoot occasionally. It's about a 2.5 inch bore. Makes a lot of smoke and the lead ball penetrates like crazy in a dirt berm. Old lead bullets picked up at the range make interesting cannister shot too. He uses old plastic jars or tinfoil to hold the shot for loading. Makes a mess of a refrigerator carton if aimed right too.