On this photo (in a generally-positive Wired piece on the AR-15 originally linked by Unk).
Image, supposedly from 1967, shows a soldier in what could be 1960s Army uniform, laying in "jungle" (looks like North American woods to me), holding a rifle described as an AR-15 (!), which has an adjustable stock and a suspiciously ribbed and non-tapered foregrip. I'm thinkin' Airsoft. Seriously, you .mil folks and fans, terminology aside, when did the modern adjustable shoulder stock come into use?
It's an AP photo, so I'm not quoting it here (they have a little problem understanding "fair use"); that would account for the ignorant caption. But the photo itself...? Compare the rifle to others shown on this page of AP pictures of '67 Vietnam troops.
Hoax or time travel? Yeah, that's the question.
ETA: PDB and Frank W. James bring the win in Comments: probably an XM-117E2 or E1, an astonishingly modern-looking full-auto variant. AP's caption description is still wrong, of course.
INDIANAPOLIS HAMFEST, 2015
4 months ago