It doesn’t matter why. What matters is at the time, the troops in the field clamored for the return of their trusty M14’s.you know that only happened because of political pressures and idiots in the field saying the rifle didn't need to be cleaned along with the ammunition manufacturer using a different powder which caused more fouling combined with the humid conditions which didn't help, but no, you can still parrot fuddlore and myth to hold onto your fossil of an old rifle that the Italians still made better.
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By the way, the M16’s main failures were switching powders (the switch to ball powder was a huge fiasco) and increasing the twist rate unnecessarily. But the point is, at the time, soldiers and Marines would have rather had their hands on an M14.