The Vietnam war saw many upgrades in the weapons used in combat than they did in WWII. WWII weapons were good for their time, but were still difficult to manage and were sometime heavy an inaccurate in firing, like Browning Automatic Rifle (BAR) light machine gun. It was very useful and reliable but had a heavy recoil, long length, and heavy weight but it was so powerful because it shot the heavy hitting .30-06mm bullet. But eventually, in times of desperate situations, difficult to manage weapons like the Thompson and BAR were made obsolete and needed rapid replacements for a continually growing power, the U.S.
First to be mentioned is the submachine gun (SMG).