When I posted the finished pics of the Land raider Redeemer I did as a commission, I had a few requests for how I got the gun barrles to look the way they did.
Real easy to do.
1. As an example, I'm using this heavy flamer. I prime/basecoated it black. It's part of the same commission and is going on the bottom of a Landspeeder so I want it to match everything else.
2. Paint the base colors, in this case, it's only silver.
3. Heavy wash of Badab Black over the silver.
4. Highlight the black portions of the gun.
5. Heavy drybrush of brown starting at the muzzle and going back about 2/3 of the way of the vented portion of the muzzle.
6. Carefully drybrush black on just the front 1/3 of the muzzle, you want to see some brown before you get into the silver on the back part of the barrel.
You can vary the depths to your liking, the heavier the soot, the more used the gun looks.
And you don't have to worry about how "clean" you drybrush... there can be some irregularities in the buildup. Just make sure you don't leave any hard edges with your brown and black, make sure to fade them in the small space you have.
UPDATE: If you want to add that heat discolored look to your metal gun barrels, this post here will help.