Probably the Sniper-rated Model K44 (Vietnamese variant of the Mosin-Nagant M44) 7.62X54R
It looked .30 cal/7.62 to me
If anyone had known the range to the VC sniper, they probably would have popped him first. If the range were mentioned, I've forgotten it over the ensuing 48 years. Probably 300 yards +/- 100, but maybe up to 600 or so. Who knows?