I use a lathe to turn some thickness from the rim, cutting on the inside so the primer pocket stays the correct depth.

I realize it's been 5 days or so since you posted, but if you've not solved your problem here's an idea.  How about making a mandrel from wood (or metal) that's a fairly snug fit inside the brass case.  Use a small hose clamp to tighten with part of clamp on brass, other part on mandrel so the case doesn't slip.  Next take a file with the edge ground smooth so it won't mark the case, and file the inside of rim with drill motor (hand or drillpress) spinning the mandrel.

Seems like that would work?

Putz'n in the kitchen, making BBQ beans from a dry package of Great Northerns, they will simmer all day.

Morning y'all.
Coffee and tea are ready.

'Tis 32 and sunny. High of 62.

Slim

Here's a direct quote from Captain Dennet, "Why didn't they take the tank".

I've run many thousands of rounds of BP 44WCF ammo through my 24" Winchester/Miroku rifle with uniformly excellent results.  My standard load is 32 grains of Olde Eynsford FFg under an Accurate 215 grain bullet.  I'll shoot a 2 or 3 day 10 to 12 stage match with no cleaning and zero problems.  Clean it afterwards with 4 or 5 patches using moose milk and a pull of a bore snake.  The bore is still shiny.  I'll never foul it's barrel with the new fad smokeless powders!