.280 Enfield...Ahead of its time

[LRRPF52]

There is zero need for a 5.56 NATO rifle, since a 14.5" or 16" barrel will give you all the velocity you need.

For the MRMG (Multi-Role Machine-Gun) load, there is plenty of room to play with for weight in the high sectional density of 6.5mm projectiles without magwell restrictions. Here's an extreme example of a very long Japanese 6.5mm projectile: