New Cartridge Developments and Implications for Dismounted Infantry Soldiers

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What are the upper row of belted cartridges in the last pic? They look considerably skinnier than the bottom row.

Maybe for a rifle/carbine they could take advantage of the shorter overall cartridge length and use a P90 style of magazine. I don't know how the ergonomics would work out, but if possible it might be advantageous. Lying prone with a 50 round magazine full of a flat shooting 6.5 cartridge would be pretty cool.

Maybe a P90 type mag underneath the bore. It'd shove sideways into a slot, and be ejected out the other side. The bolt would have longer travel (long recoil) and still bottom eject. The magazine slot would have a lever that ejects the mag out the opposite side from it's insertion. It could possibly be partially ambidextrous, by allowing a reversible catch system.

Getting the mag under the barrel would eliminate the width problem for getting your head closer to centerline, and also get the optics closer to the boreline..... Just some more goofy out loud thoughts.