Well, since I only have 2 ML out of 8 that have removable breech plugs, I have to leave them in. IF I were putting one into "long-term" storage where I could not check the bores every couple of months or so, I would make sure they are perfectly clean inside, then use R.I.G., (RUST INHIBITING GREASE) just as I do withmodern rifles.

If I can swab them out every three months or so when not in use, I use Birchwood-Casey Sheath on a saturated patch, and libarally wet the innards of the gun with this stuff. It absolutely prevents rust!