The best bet is with diluted vinegar and a rag. Take the rag and soak it with vinegar and wipe the bluing with it and try to get the most vinegar where the gun would show the most original wear. Wipe with a wet cloth and see what the effect is. You may have to leave the vinegar on longer to get the desired effect, but don't leave it too long and ruin the looks altogether. When you are finished, be sure to spray with WD-40 and wipe it down completely so there is no continued stripping of the blueing.

[Linked Image from i.postimg.cc]