I paint them black, then do a black wash on it, on the bigger guns you can drybrush on some FW black powder on the tip of the barrels.
If you want to paint the tip as they were firing, then I would paint the tip dark red, red, red orange, then a red or orange wash, make sure it only apply to the aiming or assaulting guards, leave the at ease and other guard's gun tip black.
ive noticed in the only war book that the tips of lasguns barrel have a red filtered lense on them... i think thats what he may mean.
Ah, I haven't seem that. If that be the case Chris then you would just need to follow this process nut with red, however you wouldn't need as many steps. Just a dark red base, a lighter corner highlight and them a final dot of white.
On a different note, I really need to do more tutorials.....
I do quite a lot of lenses in different colours, so next time I do some I'll put them up as examples if ebveryone wants. As Cosmic has stated though, it's simply a matter of changing the colours that you work with, as the technique stays the same.