Brian,
I noticed no one answered with a reamer. There is another way you can do what you need. If you get a common counterbore, o can make a plug that fits the chamber and centers the counterbore. If you choose one a little larger than the rim recess and grind it to size, that will remove the small radius at the end. Since a counterbore cuts on the end, grinding it to the required diameter won't stop it cutting. If you turn it by hand with a tap handle and check it frequently, it's not likely to cut too deep. If you don't have a tool post grinder, you can easily "jury rig" one by stacking 3 or four cut-off discs together in a "motor tool" hand piece held in a boring tool holder, or a motor tool screwed into a steel bar (many nose caps are threaded 3/4" to hold the two sides of the motor tool together) held in the tool post. If you don't have either, the local gunsmith or machine shop will. Good luck.
Mike
