I often read of soldiers in Vietnam being coached to only load their 20-round M16 mags with 18 rounds to prevent jamming.
Exactly! It wasn't because of spring fatigue, but because of increased spring pressure with the 19th and 20th rounds. With a full 20-round mag, there was a much greater chance of the first cartridge jumping out of the mag, upon rapid insertion, and causing a jamb in the M16.
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Tonkin Gulf Yacht Club, Tết Regatta