I've done the shotgun primer conversion on some hard-to-find BP rifle brass.
This only works if you have solid head brass cases, not the old balloon head cases. The pocket has to be tight to seal against gas leakage and to resist the powerful hammer strike of the Remington Rolling Block action.
You need specific sized drills, or preferably, reamers, and countersinks to do the primer pocket sizing to fit shotgun battery cups. It is best done using a lathe and suitable collets for holding the cases, but can be done on a drill press using a cartridge case holder using a chambering reamer on a block of aluminum, and then partly sawing it lengthwise.
It's a lot of trouble to go to, but if expensive or hard-to-find cases are the only alternative, what else is a reloader to do?
