Neat gun!

As 400 says, slug the bore.

Buy some teardrop-shaped, pure lead fishing weights and select one that is just slightly bigger than the bore, and run that thru with a brass rod. Get the brass rod from a machine shop or welding shop. A 1/4 inch rod works great. You can cut a short 1 inch long piece to get it started with a hammer from the muzzle if you like. Mike both land and groove depth so you know.

Your "drop test" pretty much tells you the thing wants rims.