Just to clarify, the pictures that Spartan198 linked to are a mix of Vietnam locally McGuyvered 5.56mm mags, and modern mags for 7.62x39mm AR-15s. If you compare the angle of the mags in the Vietnam pics to those of modern 7.62 guns you can see that the angle is a lot shallower in the Vietnam pics, indicating the use of the less tapered 5.56mm round.
The idea of these extended mags was that you had this in your rifle with 30-40 (depending on exactly how it was modded) rounds in case of ambush, whilst you carried regular 20 rounders in your webbing, what with there being no suitable pouch that could carry the hybrid mags. As these were all locally McGuyvered without proper tools I imagine the quality and reliability varied greatly, but there wasn't initially any option other than the 20 round mags so they made do with what they had.
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