It's spot on for many of the patterns of Howda... As pointed out, maybe the inside of the chamber has some sort of a shape? If just straight, it could well just be a shotgun or a simple straight walled pistol cartridge. just look through towards a decent but not too bright light and see if there's a taper from the base of the breech to a couple of inches forward... Maybe take a pic from the breech end towards a decent light. Not too bright though...