1. The XM21 was a heavy barrel match M-14 rifle built by Rock Island Arsenal for the military. They originally build about 1,400 of them for use in Vietnam. In 1971, it became the M21, and was the primary US Army sniper rifle until 1988.
The M-21 did not have a heavy barrel. It was only an M-14 that exhibited better than usual accuracy over other M-14's and was then selected to become an M-21. The selector was removed and a scope mounted. A cammed Redfield scope was mounted on the one I had. If I recall correctly the cammed scope permited accurate fire between 300 and 600 meters.
The m21's that were our issue in the late 80's were manufactured at the rock island arsenal. They had the same barrel profile as a my national match M1A (I think that it was the same weight as an issue m14 barrel, but I only held a issue m14 for a day or two in ranger school) the rifles were glass bedded and easily MOA or better rifles.
The scopes were Leatherwood ART-2 scopes, which adjusted trajectory from 300-900 yds.
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