Ive been watching vids on the new US military calibers. Namely the Fury in 6.8 and the 6mm Arc on a 220 Russian parent case. Ive always wondered why they just didn’t neck up the 5.56 to either a 6mm or use an existing case with proven ballistics in the 25-45 sharps which is a 5.56 necked up to a 25 caliber you don’t loose ammo capacity like in the Arc and no added weight like the Fury.d .A barrel change is all that is required to run the cases.

6mm Max

https://www.sunnshadow.com/post/6mm-max-a-comparison-of-top-performing-small-frame-cartridges-and-ballistic-data

6×45