The best book on them is Peter Senich’s ‘The Long-Range War’ (1996). It’s the full Vietnam era history of them as ordered via Project ENSURE 240. Here’s my XM21 replica on top, and mid-80s M21 replica on bottom. The www.m14forum.com has a bunch of detailed links on the XM21.
Ok. Many old Army AMU guys like “Hook” Boutin who built them circa 1969-70 have since passed on, but if you have a specific question I’ll give it my best shot based on reference books, and if don’t know the answer, I’ll let you know.
Love my XM21s. I have all 3 gens. M84 in a G&H mount (scored one that x’ed out the M1 serial # and stamped the M14 serial # on it.
Artel version and ART II version.
If anyone needs a foil sticker for their ART II, let me know, I made a few May have some left.