M1D scoped Garands were used in Viet Nam, at least early on. My primary reference book on this is at home, but I have confirmed my memory online: scroll through this to M1D section
http://www.thegca.org/glossary-terms2.pdf
EDIT--Official Army Sniper Rifle until the Mid 1960s. An early Viet Nam setting would be perfect.
Edit, and a picture since you'll have to come up with a flash hider and the scope, I should make sure you know what they look like