Opening a can of worms. You will get 20 replies. 5 with a technique to do it and 15 arguing weather or not you should do it.
Allen Martin suggests:
Brown the barrel like normal but do not card or remove scale like you if you were making a even brown . What you are looking for is the pitting . I normally I leave my barrel in the damp box for about 8-12 days . The barrel will look ....rough !!!!!!
Some folks use bleach but I feel the pits get to large ......... and uneven .
Step two ....
At this point, sand all the browning off to the bare metal, but not deep enough to sand off the pitting. I sanded down to 320 to polish the sanding marks out. It's very messy job . Using 100 grit sandpaper going to a scotch-brite pad , I also you use hot -soapy water and wet sand .
Step three .....
Apply cold blue . make it as black as possible.... Brownell's 44/40 works for me ..... Rub the cold off with steel wool or a scotch-brite pad & oil to get the desired gray color, leaving the corners and areas around the sights dark, and remove most of the color on the high points, barrel edges, etc.
And that's it !!!!!!!!
Simple takes time and it's messy .Every job turns out different ..... but thats the beauty of it !!!!