Hal,
Some people use parafin( avaliable in canning section in gro.store)as a quick chambercast and this would be better than plaster of paris.When I was a young lad,melted sulfur was used. Of course,cerrosafe is best(it is more precise), but parafin will indicate case length and which case head family the cartridge is in.This is for the rifle, others have addressed the shot shell chambers, don't forget it was proofed for 65mm shells.
Mike