LOL Bro, literally whatever camo paint was available at the PX, local hardware store, or Walmart. Krylon, Rustoleum, Hunter's Specialties, anything.
TurdFerguson,
@Trigger Monkey has the correct answer, probably from firsthand experience. But the Army's "book answer" (the answer you are looking for) can be found here:
This is the official "Notes for Soldiers – Weapons Painting 101," a shortened version of "TACOM Life Cycle Management Command's Maintenance Information Message MI 10-040, Camouflaging Specific Small Arms.”
The instruction at the link is from 2010, and applies only to the M16/M4 family of weapons. But it shows what the Army was using to paint weapons at the time, and the procedures they used to do so. I wouldn't be at all surprised to learn that there were units that never saw the document but figured out how to paint their rifles anyway. Incidentally, Citristrip works well as a pre-cleaner.
For an extra touch of period authenticity, here's a guy selling the military surplus mesh laundry bags used to make the pattern on the rifle pictured a few posts up (post #872):
No draw string.
www.ebay.com
After the above link expires, I don't know any other sources for the bags, but it seems the Brits have a number for sale, and there was enough interaction with deployed coalition units that a British bag could conceivably have been used in theater. Or an OCD rifle painter could find something that duplicates the pattern in the photos I've posted below.
But again, any paint and any type of mesh that existed 2010 or earlier would be "authentic." Fishnet stockings from a Fayetteville dancer would be a nice, probably authentic, variation.
I lived this period, deployed as a non-sniper infantryman to both Iraq and Afghanistan, and I never really paid attention to these details while I was there. But it's good to capture them now. You've likely seen the crazy debates about what's properly "authentic" regarding various WWII rifles and gear, and that was just 75-ish years ago, with veterans still alive who fought there.
Anyway, hope this gives you what you were looking for.
Best,
Trog