What it boils down to is politicking between different departments in the Army. It's worth looking into beyond this post (ignore CadianConscripts. Guy doesn't know what he's talking about), but in the tests of both the M14 and AR15 were rigged.
The M14 had to go because it was unsuited for jungle combat (And really, was based on an obsolete combat model). But the Army Ordnance Office just had to have a select-fire rifle with a 7.62mm round... no matter how hard it was to control in fully automatic fire.
The AR 15 had it's problems with reliability issues, but those were fixed in the first few years, when the right propellant was issued and troops were trained in the appropriate care of the new rifle.
Quirel
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