Originally Posted by Jyezahn

The most common way is to soak the barrel in vinegar for a few hours. (plug the openings of the barrel before you put it in though) and use a Scotch-pad to remove the rest of the bluing. I used my Dremel with a wheel that resembles a Scotch-pad to really clean it up after soaking. Same with the take-down lever and slide-catches. (didn't soak those, the bluing comes off rather easily)

Wipe them with a little bit of oil afterwards, and voila!