Bear in mind, the guy holding the rifle is not a cutting-edge guy, but an indigenous ally.
Generally, unless the locals can buy their own weapons (think Kuwaitis), their special ops forces get standard US weapons. True then, true now. Simplifies logistics and training. If some engineering or test organization brought a prototype to the war zone –– like, say, the XM25s in Afghanistan now –– they wanted to know about its performance in combat. That means whoever got it would be an American, who could either write a report, or at least get debriefed in English. (The guys testing the XM25 are being exhaustively debriefed). An ARVN officer is a most unlikely candidate for a US weapons test. Not to say that some weren't brave or loyal, and they all had to deal with a war with no DEROS... Marvin was home. He had nowhere left to go... except to the USA in '75 if he was very, very lucky, or to reeducation camp.
I tried another angle by looking up who was at Polei Kleng. I knew two men on the list, and tried to make contact with them. One is off the net temporarily. The other had no opinion on the weapon. The guy I reached was there much later (1969) than this photo which appears to be from 1967 approximately, and he has guesses but doesn't know-for-sure any of these men.
The camp was open and staffed by USSF from 1966 to 1969 inclusive, during 1970 it was handed over to ARVNs. During that time about six or seven captains and maybe as many as ten to twelve lieutenants served there. (There was one captain slot and one or two LT slots on an A-team in this area at this time). Various guys I know have maybe-possibly IDs on the two USSF officers and a very low confidence level ID of one of the NCOs. No one thinks the SFC is Walt Shumate (but he could well have told some sergeant major he was, once - inside joke).
There is a publicly viewable site with an aviator's photos of the camp. He does misidentify the camp (as you can see from the photos of the airfield shot from inside the camp). The pentagon-shaped fort on the map, south of the village of Polei Kleng, was something else, maybe a 4th ID firebase. The camp was on the floor of a wide valley. It was a fortunate camp with few USSF KIAs.
Sorry I did not move the mystery of the gun along but I thought you would be interested in some of the historic context to the photo. One more angle that deserves investigation is the source of the photo –– the photographer seems to have been a professional with a wide-angle lens, a pro's tool. This could mean that he was a newsman, but it might not. (The Special Forces O&I school included an intensive class on photography, so at least two or three of the guys in the camp would have been adept photographers, and the armed services had their own journalists with first-rate equipment –– still do).
(For the record, and to forestall any confusion, I am not a Vietnam veteran, I came along later. Vietnam vets were my mentors).