Just as a hypothetical….

AR-57
BUT
20″ or 22″ barrel.
This would give the room to extend the rail all the way forwards
THEN
you could take two 50rd p90 mags and join them with plastic welding to make a 100rd mag.
THEN
you could have the gun also be able to accept 50rd mags because the rear loading port would be the same, and you could simply have a notch for accepting the forward locking piece at the 50rd location as well as the 100rd location
BUT ALSO
with the rails on the side, you could have 100rd mags as well, stored on each side of the gun if you made brackets/mounts for them
300rds of 5.7 is only 4 pounds
so you could have a gun with 300 self contained rounds on it, not including any 50rd mags on your person

…..

wait a minute guys wait a minute wait a minute

5.7:

50 AE:

Did some modeling and…..

50 AE *should* fit in the body of a P90 mag…..

Singlestack, but still….
Imagine a full auto P90 chambered in 50 AE
You would be able to fit 14 rounds of 50 AE in a standard length P90 magazine

Maybe 13 after adjustments to the feeding mechanism