I am no rifle manufacturer, but here's my thinking:
1. Larue already makes tubes with 30 cal holes in them.
2. Larue already makes .223 face bolts.
Buy a 300 blackout reamer and you are set!
6.5 Grendel=Russian bolt face, and .264 tube
6.8 SPC= 30 Rem AR/10mm pistol bolt face and .277 tubes
6.5 CM=308 bolt face(Hey we already make a 308 bolt!)
but, we don't make a .264 tube
338 Fed=308 bolt face(Hey we already make a 308 bolt!)
but, we don't make a .338 tube
I already have a 300 blackout, but it makes the most sense to me!