There were a number of weapons used experimentally by small units of US military forces in Vietnam. One of the better known is the Stoner 63 system (which we will be uploading some material on shortly) – and one of the lesser known was the Harrington & Richardson T223 rifle. The T223 was basically a copy of the H&K33, a full size roller-delayed rifle in .223 caliber.
We have a preliminary operating and maintenance manual for the T223, dated June 1965. One of the advantages the rifle had at that time was the use of 40-round magazines in addition to 20-round ones. That early in the Vietnam war, M16 rifles were only being supplied with 20-rounders, and 30s were rare and difficult to find. For special forces units, the additional magazine capacity was a definite boon.
(1965) H&R T223 rifle manual (English)