The gyrojet.
A whole new proposed branch of firearms which looked to replace bullets as we know them with miniature rockets (.50 cal-ish from memory). A whole lot were privately purchased to take to Vietnam apparently, however, being a rocket, their launch velocity is incredibly slow and they take a while to get up to speed - not so useful in close jungle, not to mention issues with moisture and rocket fuel.
Apparently they were just a lemon concept. https://en.wikipedia.org/wiki/Gyrojet
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