From The American Rifleman,June 1971, p.62,"Questions and Answers":
Long Magazine For M16 Rifle
The AK-47 assault rifle, used by the North Vietnamese and by many of our South Vietnamese allies, has a 30-round magazine which is everywhere considered an important feature. The ARVN soldiers here in the Mekong Delta attach an AK-47 magazine to the bottom of the M16A1 rifle magazine, doubling the M16A1 capacity from 18 or 19 rounds (you never put in 20) to 38 rounds. the attachment is done by a flanging operation, done with only local tools. Welding or soldering would be desirable, but neither is available to the soldier in the field. The beauty of this modification is that it really works. This is my second tour as a U.S. adviser in the Delta and I am continually astonished at the ultimate utilization by the South Viet Nam soldier, who lives a hard life completely without frills and material wealth, of what he does have.
Answer: That account is indeed interesting and revealing.
A 30-round magazine has now been adopted for the M16A1 rifle. The following summarizes the development of this magazine.
The 30-round magazine was a component of the XM177 series of submachine guns developed to supplement the M16A1 rifle... The XM177E2 was given extensive engineering and service testing, including use in Viet Nam. In July, 1970, however, it was dropped.
The XM177E2's 30-round magazines could be used in M16A1 rifles, and were so used by individuals in Viet Nam who could obtain them by any means. After what has been described as a difficult period of development, this 30-round magazine was adopted for the M16A1 rifle in 1970, and is being produced for that use.-E.H.H
it sounds like the urban legend that existed back then that the AK47 could fire M16 ammo also!