I did some additional checking on the Marlin Ranger 103 that you mentioned, and it only has one lockup bite at the lower rear of a large recoil lug. I still do not recommend using it as a donor gun for any kind of rifle cartridge. Sorry. If you want an over and under donor for conversion, look for an Ithaca 500 with rusted, bulged, or blown barrels. You should be able to find one cheap if you look long enough. That gun has the Kirsten double bolting system that is one of the strongest on the market. Bob