Paul, I'd echo what Ron said, basically. I re-did two A&H that were in good shape, and a third akin to what you described. Scrubbed, bore pasted, soaked that one over a couple weeks until it stopped flash rusting. Then dyna bore coated all three.
What I found was, none of the bores rusted while unfired with bore coat, one was five years later! Probably not recommended, but hey. Once fired, the two nice barrels remained easy keepers using blackthorn 209, as you might expect being they were maintained going forward with petro based cleaning. The third, rough barreled gun, I shoot only goex FF in. The rough areas in the bore will rust superficially like any BP fouled bore if it sits fouled all day. The rest of the bore may rust a trace here and there, not much. I didn't have good luck preserving that bore between uses using ballistol, and am generally a fan of the stuff. Did find that sheath/barricade works like a charm for that tough bore. I just patch it out before shooting and hasn't been an issue.
Sum bean shoots sabot prbullets with aplomb though, imperfections and all.
