Even though you've already gotten it done, burn marks are quite similar to rust, as far as painting goes.
I usually start with a dark brown for burns-- one that's nearly black. Then I stipple it with a mix of the dark brown and a dark red (GW scab red isn't a bad shade for that). From there, it's just lining the area with various silvers so that it looks suitably charred, and maybe even a little warped. You just want to make sure the dark brown shows through a tiny bit around the inside of the barrel and the edges, and around any exhaust vents.
When i want a good residue on my gun barrels i paint on a layer of black and while it is still wet, sprinkle some baking soda on it and let the paint dry. Then paint a bit more black over the soda. Great powder style burns.
for burns on metal, (specifically melta barrels and exaust ports) i like to prime black, and liberally drybrush boltgun then I lay down a layer of a custom mixture of inks (25%black/75%red) and I layer it about 1/3rd more the distance that I want the actual scorch, then I let that dry completely and then drybrush black over the section where the carbon residue is (about 2/3rd the area that was inked.)
I havn't tried it yet, but with blue and purple inks tossed in in very thin layers/slivers, you might be able to get the overheated rainbow metal effect.
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