really easy but effective tip: painting a gun barrel (or an exhaust tube) in a proper way to represent a burned part is much cooler to see than a gun with a plain metal finish so i'm gonna tell you how to achieve that without much effort. we're gonna pass through 4 steps:
1) wash the gun with a really watered down Snakebite leather
2) wash a smaller area with Bestial brown
3) now do the same with Scorched brown
4) last: give the barrel a Black wash to make everything homogenous
and there you go!