Make a "stopper" for the muzzle end - a length of wooden dowel (or something) in a length of automotive heater hose that slides over the muzzle end - tighten down with hose clamps around both the dowel and the barrel. Don't really want a plug up inside the muzzle end - want the "juice" to work right to the very end of the bore. Set barrel vertical in a soft jaw vise. Fill that bore completely with 50/50 Acetone + Automotive Transmission Fluid (ATF) that has been mixed thoroughly (I use a plastic pop bottle for mixing / shaking up). Lay something flat over the breech end - the acetone will want to evaporate if left open to air. Let soak for a day or two. Will penetrate and loosen up any rust or corrosion. Then scrub well with a bronze brush, and follow up with several patches, until they come out "clean". Will be as "clean" as you will get it. Then shoot it - don't condemn it just based on what the bore might look like - actually shoot it once all the loose crap and corruption has been removed and then decide, based on bullet holes in target, whether it is to be retired or not.