If there is a double strike then that would pretty much solve the ID. Most people don't realize just how hard it is to positively ID a shell casing. That's especially difficult if there is no writing or markings on the case. The reason that is a problem is because there may have evolved 40 or 50 variations from one parent case. They are shortened, necked up or down and vary slightly in makeup depending on the firearm for which they are intended. The variation from one case to another although completely different may be only a 16th of an inch or so. Then with the advent of smokeless powder came a whole new batch of cases even further enhanced and changed to meet the higher pressures generated by smokeless powder. That's why I find the history and evolution of firearm ammo so interesting. Monty