And not wanting to get into caliber discussion here, but a 6.5 Grendel is quite close to a 7.62 NATO. Might well go for a single caliber then. But also the more AK-47 style 6.8SPC for assault carbines and the 7.62mm NATO for MGs plus DMR should be considered.
This would only increase overall weight carried. 6.5 would replace both 7.62 and 5.56, or maybe even only 7.62, and therefore save weight for near equal performance (and that would need to be tested and proven of course). 6.8SPC cannot replace 7.62 because it does not have the range.