Many years ago, I was approached by a gentleman who wanted some 12 ga brass cartridges loaded with bullets. I cast up some .820 (8 bore) round balls and purchased a couple of boxes of 12 ga brass cartridges.
I had a 1/2 or maybe 5/8" thick iron plate which I drilled a hole of the correct size in and with an arbor press, I pushed the .820 balls through, then pressed them into the casing.
I imagine the same thing could be accomplished with a heavy duty drill press if you don't have an arbor press.
Obviously, the projectile does not have grease grooves, but I think the same thing could be accomplished with prelubed 12 ga cushion wads. Bonus, you would not have lead fouling in the bore, probably a bit faster muzzle velocity, and loading each shot would wipe the bore clear of powder fouling.
Here is the finished cartridge.
John (Bigsmoke)