You might want to do a search here, this has been discussed in the past, so should have some good info.
Cold blue the barrel, plug the muzzle and vent hole, then immerse the barrel in a Clorox. A 6" pvc pipe cut in half, length wise, with caps on each end will work. As others have mentioned, the process can go fairly fast. It's been a long time since I've done this, but if I recall, I started checking after ten minutes or so. Once you have the desired effect, card off all the gunk with course steel wool and naval (sp?) jelly, then rinse in water. Best to do all this outside... After all this, you can re cold blue, then rub back with steel wool and some motor oil or WD-40. The blue should stay in the pitted areas, and the rest of the barrel will be a greyish color. Hope that helps and good luck.
Ed