In the late 50s to mid 60s there was a movement to de-emphasize induvidual acurracy (they assumed that city boys couldn't shoot straight), and try to put multiple rounds downrange in as short a time as possible, to get a sort of "shotgun" effect from a small caliber rifled barrel.
The best of these designs was the SPIW as designed by H&R, which shot sub caliber flechettes from a .22 cal barrel, with a 30mm grenade launcher which besides grenades also fired ammo similar to the 12 ga rounds available now holding 50+ flechettes.
There were problems moving into the production phase as we were increasing our invovement in Vietnam, and Colt/AR had more pressure on certain members of congress & the military procurement system, so the M16 (which was designed as an Air Force survival weapon) was adopted by the Army & the SPIW program was shelved.