What you really need is Tamiya Clear Blue and Clear Orange
Lay a thin coat of clear Orange over 2/3 of the barrel,Blue on the 1/3 at its tip.
Overlap the orange and blue where they meet.
The further away from the heat source the more orange it gets.
Tamiya makes a weathering powder compact with burnt blue and burnt orange that work really good with smaller stuff ie lascannon barrels
Here is a video of it in action
http://m.youtube.com/watch?v=XVjjvSaQeew
MIG also makes Burnt Blue