I would disagree with you that FMJ is not a great Vietnam War movie. In fact, I would say that it is one of the greatest war movies of any war.
There is a quote by film maker François Truffaut that basically says you can’t make a anti-war film because of the spectacle of violence they portray. Indeed most war films conclude with some epic final battle with the protagonists either surviving to be transformed by their ordeal or going out in a heroic blaze of glory.
FMJ is one of the few films IMHO that portrays the true futility and drudgery of war. Kubrick turns several war movie tropes on their head. Joker’s first taste of actual combat is the Tet Offensive Viet Cong raid on his base. Unlike earlier films like Platoon where the battle is a desperate Alamo-like last stand against overwhelming forces, this raid is largely ineffective. But we get to witness their fear as all their bravado and big talk in the baracks suddenly disappears as they realize they are under attack.
Perhaps emboldened by surviving the relatively safe and anonymous combat of firing an M60 from a bunker at shapes 300m away, Joker is eager to “get into the shit” at the Battle of Hue. But there are no typical war movie heroics to be seen. On the ride out on the Sikorsky H-34 Choctaw, Joker and Raptorman meet a presumably psychotic door gunner who is easily able to kill women and children by “not leading them so much”. Later they encounter mass graves filled by the NVA with civilians and deal with egotistical and beurocratic officers in the rear. There is no heroic charging of machinegun nests or jumping on grenades to save their squadmates. Just the monotonous grind of sweeping across the destroyed landscape as one by one, various characters we meet are unceremoneously killed by mortars, snipers and booby traps.
It is in the final act with the sniper where we experience the ultimate horrors of war. All the tanks and helicopters and technology of war are now far away and useless. They might as well be on the moon. We are left with a small isolated and confused squad of lost Marines getting picked apart by an unseen sniper in the claustrophobic rubble of Hue. And when they finally rally a counterattack, it is not some satisfying storming of some bunker complex manned by a company of hardened NVA or Viet Cong. It’s just a single girl with a rifle.
IMHO, that’s what makes FMJ a great war film. Kubrick methodically strips away all the hype and heroic spectacle of most war films and makes it into a more intimate story of survival.