Around the same time that Small Caliber High Velocity was starting to percolate through the US Military, so was another set of programs designed to increase the hit probability of the individual soldier. Considering that 5.56 was ultimately adopted as the standard loading does not take away from the ingenuity of the designers at the time, who developed many remarkable solutions.
Various items proposed to increase hit probability included using multiple projectiles including rapid fire (which exists today in the AN-94) and shooting multiple projectiles from the same detonation in the same vein as a shotgun, just with the projectiles stacked on top of one another.
To me, one of the most interesting concepts was the use of flachettes, which are basically ballistic darts. In the case of the XM-19, the dart was accelerated to over 4800 fps. This high velocity caused its own issues, but the idea continues to be of real interest to weapons designers, especially those looking at underwater weapons.
The magazines were an interesting design. Rather than use the standard shaped wire spring common even to today, the magazine used roller springs, which provide constant tension. Coiled wire springs provide more force when compressed as most shooters know when trying to put the last rounds in a magazine.
The design was ahead of its time and even has recently been reintroduced to the MSR through the new TorkMags.