That was almost certainly because of issues with maintenance, ammo, etc. Stuff that's long since been resolved with the M4 / M16.

Yes; when the M16 first came out, it had some teething issues, but again; almost all of it was due to improper or poor cleaning, maybe magazine problems, and the fact that the military did some sort of ammo change, without fully realizing / testing the impact on real world M16's.