When I qualified in the Air Force, 1966, I used the first model M-16 without the bird cage suppressor on the muzzle. It worked fine. No problems. I also remember it threw it’s empty cases forward.
Three years later I qualified again. No problems.
Maybe we were using early ammo before the change to ball powder caused the problems.
The first evaluation M16s were according to Stoner’s specs. Then the Army changed them for the general issue to grunts in Vietnam, along with dropping the specified powder for the ammo, and used leftover WW2 powder (which fouled the mechanism) .