As originally issued would be 20rd.
Current issue is 30...technically, high(er) capacity.
The XM16E1 entered US military service in 1964, and In March 1965, the Army began to issue the XM16E1 to infantry units on a trial basis.
In February 1967, the improved XM16E1 was standardized as the M16A1, and the 30 rd mag went into production and issued.
In 1969, the M16A1 officially replaced the M-14 to become the U.S. military's standard service rifle.
I consider either magazine as "normal" capacity, as issued, starting in 1967 once the M-16A1 was standardized.
BTW, the original ArmaLite AR-15 was developed in the late 50's using a 25 round magazine.
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