Back to the rifle, though (the top one, though this might have some weight with the bottom one): I know I keep bringing this up, but if we take the plastic casings from the LSAT system, use 6.8 rounds, extend the length back up to 5.56 length (so we are essentially overcharging the 6.
, and use that Vector design, and we might have something.
Belt feed it, and I am RIGHT THERE.
I might also wonder about something in the .50 Beowulf range. Most infantry engagements are at relatively close range. Though I can see something with relatively bulky ammo being a special item.