This from Google. The operant word is SELECTIVE fire.

"An assault rifle---forget the AP Style Book definition, its
dead wrong---is a reduced power rifle caliber, selective-
fire, reasonably compact weapon smaller in size than a full-
caliber rifle, capable of a reasonable degree of accuracy
out to 400 yards. Generally, an assault rifle accepts a
magazine of a least 20 rounds. One can construe certain
full caliber rifles to meet this specification, but
submachine guns can only loosely border on any definition of
the true assault rifle. Beretta's M38A, the Hungarian 39M,
and long-barreled versions of the Finnish Suomi come very
close; at the other end of the spectrum, the U.S. Carbine
M1 and M2 come very close, but in fact fall into their own
very special category. In every case, the "pure" assault
rifles replaced or supplemented much more powerful
rifles...in U.S., M16 replaced M14, in the USSR, AK an SKS
replaced the Nagant and Tokarev in 7.62x54R M91, and so on."
This is from an article in "GUNS" magazine.

troll