This is how I do it:
First, I paint inside the brake black, don't worry too much about geting a little on the outside near the openings.
Second, I grind grind up some black pastel chalk (just rub the stick on sandpaper) and apply it with an old paint brush that has the bristles cut down to about 1/8 inch. using the brush I grind it in around the muzzle and vent openings.
I repeat this step as necessary, starting at the muzle and working farther back each time to get a nice fade as it gets farther from the muzzle.
This is an easy technique to learn and with very little practice you can get any look you want from "recruit" to "battle hardend veteran"