As I recollect, the initial problems in Vietnam were also ammo related. It was a whole slew of issues resulting in the usual cluster-f**k.
I knew about HK's ridiculous sales policy on this, but I could never figure out
why? It's not like they would be selling some top-secret, uber-killer, area-51 type technology to the civilian market, just a piston operated upper. You know, like the Kalashnikov has had since the 1940s.