Even though this thread was revived from the dead, it took a most interesting turn. Also, the pictures in this thread are very, very cool!
As for C7 receivers magically having no more finish and being grey after a tour... Yeah, unless you apply some pretty weird and strong liquids to your rifle and scrub them nicely with a wire brush, maybe. As a gun plumber that has been to a number of units and done many ATIs over the years I've literally seen thousands of weapons from every unit around here, that have constantly been deployed, and the finish on the rifles, carbines, machine guns, pistols and all the other weapon systems were fine to me. Sgt. Major? Got to say I haven't heard that a whole lot.
As for painting weapons, it's okay on tour depending on what your bosses let you guys do (especially for you recce and other cool doods). However, us gun plumbers do have a nice little tech update regarding this very subject (and I think a CANFORGEN?) since the war is over and painted weapons now constitutes unauthorized small arms modification unless you're actually a sniper. I personally don't knife people for this because I couldn't care less (unless it actually prevents me from doing my job, oh and that one infantry private I saw who used some permanent paint on the inside of his receiver because of his cawadoody fantasies) but I can't say the same for the others in my trade. I normally just tell whoever takes care of the vault but CSMs may not like that if they walk in there and see stuff like that. They can't say I didn't warn them.