There is an easy method that works on the most rusty relics, that removes all vestige of rust.
Get a plastic bucket, or other container that will submerge the part you want cleaned up.
Put in enough water to cover it. For your purpose, you could just have the breech end stuck in far enough to cover. Then add Arm and Hammer Washing Soda. About a pound to two gallons is about right. Stir it around to get it desolved. This is reusable, by the way, so you can keep it for future projects. Either seal it, or let the water evaporate out until next time.
Then, find a piece of steel to put down into the bucket, not touching the other piece.
Attach the electrodes from a battery charger to the piece you want cleaned up, and the other to the steel.
Run the charger on 6V, 12 will work, but not as fast, for some reason.
Turn on the charger, and watch for bubbles. You should see small bubbles coming from the rusted piece within a minute. If not, reverse the polarity. I think you need the negative on the rusty piece, but don't remember for sure. No big deal, you won't hurt anything.
For something like a rusted in nipple, this may take a week or more. But you will do no damage to the base metal. Only the rust will be removed. If you are cleaning something with surface rust, a couple days usually will do it. Take it out, and wash with clear water, dry and oil.
The metal will come out with a grey color to it, and will re-brown or blue easily.
