Agreed with other posts.

You'll find a nice Webly & Scott boxlock, typically the model 700, for a decent price. Westley Richards made a large number of good boxlock guns in all grades too. The 12 gauges are the least expensive because there are so many of them around.

Just do the usual checks of the barrels specs. and proofing. I would reload or buy 7/8 ounce loads. It's easier on the gun and your shoulder.