Make sure the locks engage half and full cock positively, with no chance of slipping or jarring, and that nothing within the locks is cracked or broken - they can still function with some parts in that condition.
Also make sure the barrel wall thicknesses are good, ribs on solidly, no deep pits etc - just as for a breech loader; many a muzzle loader has been rod-worn paper thin at the muzzles, but can still be surprisingly thick lower down.
Good luck; they're good fun, and once appropriately loaded, give nothing away in performance to any shotgun.
RG
