If it helps, there is quite a bit of information about weapons used during the Vietnam War, provided by the late Lee Russell, a technical advisor during Season 1 of Tour of Duty, on the TourofDutyInfo webpage:

Tour of Duty Info - Weapons

--Previous Message--
: Ar-15s were made by Colt Industries in
: Hartford Connecticut. During the early days
: of Vietnam War, the SP1 model was designed
: and put into production without proper
: testing in the Vietnamese climate. It was a
: failure as it often jammed during fire
: fights. Later on, the m16a1 was designed.
: This consisted of a few upgrades including a
: Forward Fire Assist on the upper receiver.
: It acted as "button" that would
: help with jamming.
:
: Later on, the smaller Xm177e1s were
: designed and given to Commando and tank
: units.
:
: --Previous Message--
: Lately I have been interested in the
: firearms
: shown in Tour Of Duty episodes and always
: thought that all M-16 rifles were made by
: Colt who bought the rights and ownership
: from Armalite and Eugene Stoner (who
: designed it). Apparently while Googling to
: learn more about the M-16 rifle it seems
: that Bushmaster also made them during the
: Vietnam war.
:
: I would like to know the facts from members
: on here about what is correct. By the way
: what is the best M-16 rifle (or AR-15 rifle)
: to own today (as everyone makes it now)? as
: I am looking to purchase an AR-15. Thank you
: and TOD is one of my favorite shows of all
: times.
:
: Sidney
: Dartmouth, Nova Scotia
: Canada
:
:
: