The current FOB event mission, Bound Dragons (now coming around for the second time, and presumably recurring again in the future) features guards who all wear riot suits with full helmets. As in single-player, There's no way to take them out with a tranquilizer weapon, not even through the small gap between their helmet faceplate and chest protection. It seems like most rifles can't shoot off their helmets, and most shotguns won't even knock them down. (I haven't developed any of the high-grade online-only weapons, but I have the highest offline grades.) This is a big step up from the typical FOB guards in Battle Dress, who can fairly easily be taken out with headshots from lethal or tranquilizer weapons.
I mostly used CQC on riot suit guards during single-player, and now I'm struggling to find what else I can use to take them out. I know they're not protected from sleep grenades. The Brennan or Serval can kill them with a headshot, through their helmet. The rocket arm can stun them (I don't know if the blast arm can kill them). And Snake's running punch will knock them out and knock off their helmet. Since I'm trying to go non-lethal when possible, I've been mostly just using the rocket arm when playing this event, but that leaves me stuck with the Serval if I'm spotted or don't have time to fire the rocket arm.
Can any other weapons take out riot suit guards with one hit, or knock off their helmet to leave them vulnerable? Non-lethal and silent is preferable (e.g. if a high-grade shotgun can stun them) but lethal is acceptable.