Military uses the 55gr.. Unless they;re special ops then they use what's best suite for them and any mission they may go or be on and it isn't always .223. Some are even using 6.8spc and 6.5Grendel even from what I've read and seen on Weaponology with Mack.
Some of which is still being tested.
If I understand it right, the lower the twist rate, meaning 1:8 and or 1:7 can use heavier bullets i.e. 75gr. and the higher the twist 1:12 1:13 use the lighter bullets i.e. 55gr.. And the ones in the middle 1:9 can use anywhere from 55gr-69 gr. Least that's what I took from he read I linked in my previous post???
Personally, with 1:9 I like and prefer 62gr..
As for that specific rifle, you'll have to try different weights with different loads to find the "best" for that rifle. Reloading will give you the best chance of doing that. Other then that you'll have to find a factory round that it likes best. And with either you will be able to tell...