Offshore,
I work on one like this a few years back that a friend picked up at a garage sale, .50 CVA Mountain Rifle USA barrel. I plugged the nipple and filled the barrel with PB Blaster and let it sit for a couple days to let the penetrating oil work it's way under the rust. Drained the barrel and ran some patches to remove the majority of the crud out of it. Then, like Birddog said, scrub the bore with scotchbrite with PB Blaster. Get the good green scotchbrite by 3M, not the cheap stuff. I use a worn .50 button jag and compress the scotchbrite and get it real flexable, 1 to 1 1/4 inch scotchbrite patch seems to work pretty good for me. About 20 or so strokes, changing the scotchbrite often, plenty of PB Blaster. Repeat the process with 0000 steel wool. The last thing I did is polish the bore with a tight pillow tick patch and Brasso, about 100 strokes, changing patches often. Then clean it. Luckily, there wasn't any pitting to speak of. With your salt air conditions, you might not be as lucky. This one ended up being an absolute tack driver; 10 shots, ragged 1 inch hole off the bench using .490 ball, .018 pillow tick patch, spit lube over 60 grains of FFFg Goex. If your barrel is lightly pitted, still go shoot it. Some barrels, even lightly pitted, shoot very well.
Rick
