Pitting can be somewhat eleviated with careful polishing with compounds and a lead lapp. The breech must be removed for this. Lapping with compounds or a lead lap smoothes and rounds/polishes the edges and helps them to release fouling when cleaning. With descent loads, it will shoot well - after polishing - most do.

Words cannot describe what the pitting is looks like - only seeing is understanding and recognising. To some people a slight roughness is pitting - any 'spot' in an otherwise perfectly polished mirror-bright finish, is pitting. It is normal for any gun that old to have a light dusting of fine pitting throughout - these can be polished somewhat and still shoot quite well - it isn't a 1,000 yard match rifle.

12 pounds is pretty light I think - meant for a load of probably no more than 200gr. That's a gun I'd like to own.