Indeed - lock plate must be suitable, however a small lock as on my .54 pistol would work just fine. It is the Bailes (historically the Studenhymer?hammer lock I think) lock, from trackofthewolf and is available in either flint or percussion. Full sized rifle locks are too long & create an ungainly piece, just like the Pedersoli 20 bore pistol.

A pair of .577 Snider rounds with flat point hardened bullets would work just fine on bears, seems to me.
A 4 barreled Lancaster Howdah in .577 Snider would be even better. (many decades ago, Guns and Ammo Magazine- might have been the annual)