[url=https://en.wikipedia.org/wiki/RPD_machine_gun]RPD FTW[/url] (RPK had just entered soviet frontline service in 1961, and had not trickled down to Vietnam yet)

The RPD has its fair share of quirks, however. I'd suggest doing some in-depth stuff on how it is loaded and carried, and compared to other firearms your character would have used.
[url=https://www.youtube.com/watch?v=UAIByp3l2Bc][1][/url] [url=https://www.youtube.com/watch?v=3vGbL45_qVs][2][/url]

The ammo-drum, in particular. Was probably the only truly unpopular thing.