I converted some 50 .303 british to .410.
I just chucked each one in a cordless drill. Had a mill bastard file. Made a safe edge to go against the brass with 2 strips of duct tape.
Only took 20 to 30 seconds to reduce the diameter and the thickness to where they would chamber. I checked each one as I did it and once it chambered fully I chucked it back up and did a little polish with a piece of 0000 steel wool. They looked good and work well in my old Mossberg bolt action .410. I fireformed and loaded mild loads of Red Dot under a .40sw cast bullet.
Some of the bases don't blow all the way out, but they still work. and it isn't like I need the room for powder.