Throughout 1962 and 1963, the U.S. military extensively tested the AR-15. Positive evaluations emphasized its lightness, “lethality”, and reliability.[17] However, the Army Materiel Command criticized its inaccuracy at longer ranges and lack of penetrating power at higher ranges.[43][37][17] In early 1963, the U.S. Special Forces asked, and was given permission, to make the AR-15 its standard weapon.
The Secret service was using this “new rifle”. In 5.56.
“However, the Army Materiel Command criticized its inaccuracy at longer ranges and lack of penetrating power at higher ranges.”
Which begs the question as to why conspirators would use an experimental rifle of dubious reliability rather than tried and tested rifle and ammunition variants?
The AR-15 was being demonstrated to the military, in .223 remington, in 1959.