I've read somewhere here that a pencil can be used for highlighting panel lines and for weathering weapons sush as machine guns. Anyone any advice on the methods and what type of pencil to use.

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I use colored pencils, as well as your standard yellow pencil. Copper, gold, bronze, and silver. Don't just limit yourself to weapons. Works wonders on worn, exposed metal surfaces and manual control interfaces. You can also do buttons, and insignias, and.....

Standard pencil lead as in a mechanical pencil. There you can get it nice and sharp with the sandpaper.

"B" weight lead should be avoided as it is too soft unless doing large areas of bare metal. Standard yellow pencil is 2H. 3H and 4H are harder lead and give more subtle results.

My standard technique for weapons is as follows:
I airbrush a coat of Model Master buffable gun metal. Next, I use my fingers and rub some Vallejo Steel pigment over the entire works. Next I buff it up by just rubbing it in an old T shirt. Then I hit the high edges with a standard #2 pencil, then apply a very precise pin wash of black panel liner to the recessed areas, avoiding any clean up-spots. Do not apply a top coat, as it will destroy all of the bright metal work you just did.

Sounds good, will try it next time. Thanks!

Not forgetting that metal parts of most most firearms start out black through being Parkerised, blued, phosphated or even painted - depending on age and source. Over time these can wear off, chip and scratch. Barrels on automatic weapons will most probably take on a matt grey appearance. In peacetime, weapons may be re-finished as part of periodic refurbishment.

There is a growing current trend towards having weapons finished in tactical colours

To give an example of the color scheme of a weapon; M240 loaders machine gun on an M1 had black plastic grip, the body was dull gun metal and the barrel was worn metal with bright silver clamps. The feed cover was painted black.

The refurbished 50 caliber M2 had olive drab painted bodies. Our ancient M85 in the M60 were typically very worn gun metal looking like the perfect example of the techniques with pencil lead.

Just remember that pencil lead must be the absolutely last thing applied to the finish. If you overspray with a clear flat, the metallic sheen disappears, and it just turns dark gray.