To my knowledge 30 round magazines were not issued for use with the M16 in vietnam. However, as the CAR-15 came into use, 30 magazines were issued: Above is a picture of an SF Medic/CIDG Company Commander, carrying a CAR-15 with a 30 round magazine. read more
I read somewhere the 30rd mag was orginally made for the early AR15/M16s, but that MacNamara chose to standardize on the 20rd mag. There were larger and longer versions of the 30rd mag that held 40rds and 50rds. Also there was a 75rd drum. read more
The first 30-round AR magazine came shortly after, a soldier with a 20-round AR magazine encountered an enemy soldier, with a 30-round AK magazine. Especially, if the soldier was being fired upon by said 30-round AK magazine. read more