During the 1950s, the US Army ran a series of programs trying to find a better solution for infantry rifles than firing single semiauto bullets. These projects (including SALVO, SALVO II, and SPIW) would include experiments with multiple barreled rifles, burst firing rifles, flechette firing weapons, and multiplex cartridges. That is, single cartridge cases with two or three bullets stacked inside. The idea was that a soldier would have a single sight picture, trigger press, and recoil impulse – but would have multiple chances to hit the target.
https://www.forgottenweapons.com/duplex-22-06-m1-garand-at-ria/
It’s a lot easier to get pictures of 5.56mm duplex than .22-06, but this gives kind of the idea. The bullet projectiles have cutouts in the rear and the second bullet is snugged up into the first, with them firing as one object when leaving the barrel.
Oh, that makes sense. Thanks for the picture.