When I worked at NWSC Crane in the early 1980s, we reworked some 25,000 very well-worn former USMC M16A1 rifles and refinished the lower receivers with a high-temperature, high-adhesion graphite-based finish. After a year or so of being repeatedly oiled [especially if 2-cycle oil or transmission fluid was used] they did indeed pick up a muddy chocolate colouration.
That rifle in the pic's fairly interesting because it's an early M16A1 [or XM16!] lower receiver upgraded with the heavier barreled M16A2 type upper. It could be that the user has a source of M855 ammo meant for the M16A2/M4 [which doesn't work too well in the 1:12 twist barrel of the M16A1/XM177E2] or it could simply be that the original barrel either wore out or got bent.
My AMVETS post has two unanodized M4s with VERY shiny lowers, and to which I've fitted 10-inch stainless steel bayonets- all for parade duty for our honour guard. The rifle in the pic hasn't been so polished, but looks to be a pretty good match for magazines that have lost their protective coating after a decade or two of use.
