Maybe some pic's will shed some light. If you can get some good pictures of the proof marks it may help. Sounds like something in the 43 to 45cal neighborhood.

If a .410 fit's nice, you can bet it's 11.15x60R. You can also shoot .44wcf (44-40) out of that.

Thomas, 11.15x60R is easy to make. Use a 9.3x74 case, trim to 2.348, and fire form. Load with a .453" cast lead bullet using 45-70 loading data.
This cartridge is not the same as thee 11.15x60R Mauser, which is a bottle necked case.

Edgar: I think I will use some 44-40 that I have which I use for Model 73 winchester. I will post some photo's Of the old girl which has a few scars but so do I. Thomas

It has a few scars but so do I.

thomas you have a fine drilling...i too like drillings i really admire the workmanship put into these guns..my drilling is a griffen made 16x16x7x57r mauser round..its a great round for deer or just about anything..that cannon yo got auto be agood for anything too....charlie

Charlie: It has great feel to it so I'm thinking I may be using it some.. Thomas

i use mine a lot.i really like these guns.wish i had one in 12x12 x 9.3x74 r...or even one in a large cailber like yours...charlie

Darn you thomas. I found myself on Guns International to day looking at drillings. I may own one shortly.

Wow...great one. I have its brother. I am thinking the its 11.15x65r though. Haven't ever shot the rifle barrel.