Obviously it's not your standard brown stock and black barrel rifle. So how should it be colored?

The stock, pistol grip, and handguards are a composite plastic material, so paint them flat black. The upper & lower receivers, barrel, muzzle, and front sight post are steel, so gunmetal highlighted with silver works. The standard-issue 30-round magazines are dark grey.

Chris

I would say black all over then dry brush the metal areas with gun metal then another dry brush with black to tone it down. Very little metal will show.

The U.S. military has not "blued" a weapon for a very very long time. So doing a gunmetal finish would be incorrect. The M-16 family are powder coated and not blued metal. The barrels were steel, while the reciever was cast,milled aluminum. So I would suggest semi gloss black for the stock and forestock. With flat black with metalizers for the high points of the metal portions.

Instead of drybrushing, one could also take a wooden graphite pencil and run over the metal parts that way. The pencil retains the grayish sheen without adding any glitter or bright silver color.

Flat black usually works well for everything except the barrel and receiver- mix up some gun metal with the black to get a dull metallic shade.

I built one of the Trumpeter rifles.

Basically painted the entire gun with Vallejo Black Primer, left the plastic parts of the gun in the black primer and took a gun metal pigment and rubbed it into the metal parts like the upper and lower receiver and barrel.

Bolt and mag I painted aluminum and then 3 or 4 coats of black wash (used more like a filter) to darken it down.

As has been pointed out, the receivers on an M16 are aluminum. Metallic is not appropriate for the pieces. If there's any wearing away at all of the Parkerization, it should be a very dull aluminum, and for only the most severely abused weapons.
Gunmetal for the barrel and FSP are fine, as well as the dust cover.

Depends on vintage, older ones are more of a grey.

http://www.retroblackrifle.com/

Gents:

Want to paint an M-16? Start with being sure what era of weapon you want and whether it is new or used or in the field or what!

The 'Nam era AR-15 / M-16 which I used and serviced as unit armorer back in the mid 70's was a "multi-media" weapon with resin butt and tapered forearms, parkerized alum receivers upper and lower, rubber butt plate, shiny black plastic pistol-grips, parkerized barrel, and varied-colored detail bits and fittings (pins, sling-rings, etc.)

The resin parts could differ in color, as forearms often got swapped and replaced in service, while butts seldom changed... upper and lower receivers on in-use weapons often differed in color. Barrels were typically a darker parkerized finish than were the receivers.

Butts and forearms were typically a soft satin in finish and NOT matt or flat black. Many resin parts were a dark greenish black rather than simple black. Pistol grips were often shiny deep black. Receivers ranged from warm gray to dull mid-gray and generally acquired a sheen from use. Barrels were darker and matt (and further dulled owing to the non-polished metal surfaces). After some use, barrels would develop a lighter dusted appearance from getting hot. Early 16 with the "bird-cage" muzzle brake often developed a metallic sheen on that brake. Later drum-shaped brakes lost finish only on the outer ring-lands. In use, all brakes got sooty.

The butt-plate was rubberized and dull and a different black from the resin parts.

I would suggest starting with a mid to dark non-blueish gray for the receivers, a darker gray for the barrel, a satin black with maybe a drop or 2 of green for the resin parts, a flat dark gray for the rubber butt-plate, satin or shiny black for the pistol-grips. Mags can be brushed aluminum, matt steel color, browned steel, dark olive steel, etc. Even parkerized dark gray. Hilite the grooves!

In service, the receivers stained and developed varied sheens from oils, etc. Barrels seldom developed any naked metal, so keep that to a minimum.

Later / more-recent versions differed, and over years the resin parts have come out with different colors. Look for pics of guns of the era and version you want to model!

Here are a few pics of earlier M-16 as reference! All borrowed from web sites for discussion purposes only!

http://upload.wikimedia.org/wikipedia/commons/thumb/6/6b/M16A1_brimob.jpg/800px-M16A1_brimob.jpg

https://lh5.googleusercontent.com/-qmKmsXFEmas/ToP6qJi9dOI/AAAAAAAAKkM/KXGXBpvyhEk/Vietnam%252520era%252520M-16.jpg

http://nazarian.no/images/wep/504_m16.jpg

http://insidethexring.com/wp-content/uploads/2013/06/M16A1.jpg

http://www.metro.us/wp-content/uploads/2013/04/M16-2-576x385.jpg

This pic is of a very well-worn issue AR-15 with M-203 grenade-launcher - the receiver wear is a great look at how these could become very worn and sheened over time!

http://blog-imgs-47.fc2.com/w/i/l/wild777/203+004_convert_20100721235150%282%29.jpg

And another, later M-16 (with forward assist...) and M-203 showing the oft-seen differently-colored upper and lower receivers...

http://stanleyarmscompany.files.wordpress.com/2009/08/m16a2m203_afmil2.jpg?w=510

Just some ideas and fodder for the ol' modelling mill!

Bob