I've read that the more refined grocery store molasses is not as effective because it supposedly doesn't have the same sulfur content as the feed store stuff. You can Google - "molasses rust removal"- for tons of information on the process. I read about different ratios of molasses to water, and settled on about 5 lbs. of dried molasses to 5 gallons of water, starting by dissolving the dried molasses in a gallon of boiling water. Some feed stores also carry liquid molasses. This method doesn't smell nearly as bad as some of the articles you read will claim, but you still wouldn't want to do it in the house. It does take a bit of patience, but I doubt if there is a cheaper, safer, easier method of removing rust. Rubber gloves are good to keep the smelly black slime from staining your skin.
I've been wanting to try the electrolytic method of rust removal that Ken61 recommends, although I have read that it is not recommended for certain hardened parts like springs because it can cause hydrogen embrittlement. But it would likely be great for barrels, because as with the molasses method, you aren't using abrasives that remove good metal, engraving, lettering, etc.
