It's not an effect you see very often or at least I don't see very often. Most of the time when we paint our gun barrels, we give them a quick coat of GW Boltgun Metal followed by a wash of Badab Black and call it a day.
Sometimes, I'll add some soot stains to the end of the barrel especially if it's a plasma weapon or a flamer. Those just cry out for the effect. Besides, it makes them look so much better for such an easy thing to do.
In this case though, I didn't want to just add some "soot" to the end of the assault cannon. It's a far too powerful weapon to just build up a little dirt at the end.
According to the fluff, they are notorious for overheating, warping and malfunctioning due to the heat created by the volume of fire it can put out. The barrels are considered expendable and changed out after each mission.
Nothing like a disposable weapon.
I wanted to make the barrels look like they've been warped by heat. Actually not warped, but the heat has been so intense that it has started to change the color of the metal. So I went looking at a few real world examples and found a handful of automotive tailpipes that had this effect.
The metal goes through a series of color shifts as it gets closer to the hottest portion. I made a few notes and after looking at some examples and came up with this chart.
To duplicate the effect, I used 4 washes and two colors.
I started by painting the barrels GW Boltgun Metal and gave them a wash of GW Badab Black. That's standard practice.
To get the heat effect, you start at the muzzle (the dangerous end) and work backwards. The first wash is GW Asurmen Blue followed by GW Leviathan Purple and then finished with GW Gryphonne Sepia. I did one right after another and blended where each color met the next. You don't need huge bands of color either, just a suggestion of the color is more than enough.
One thing you need to make sure you do is make the blue section a bit bigger than the rest so that you can go back when dry and add a bit of silver on the tip since it has gotten so hot it's burned clean.
Make sure to check out these posts as they might help:
How to paint soot stained gun barrels