Optimized Caliber

[stanc]

Not really, because the Grendel has a significant advantage in SD when using same weight projectiles. That increases barrier penetration. Since powder capacity is essentially the same, using same weight bullets the Grendel will have higher BC's and improved SD's.

Yes, really.

SPC's smaller case diameter allows higher pressure for the same bolt thrust. Higher pressure = higher MV.

Without mag length limit, SPC can use heavier bullets to match Grendel's former BC advantage.

Heavier bullets at same or higher velocity = better penetration of barriers like concrete block.

With "green" (and steel-core) bullets, 6.5 may have a practical max weight of ~120gr, whereas for 6.8 it'd be ~130gr.

Then there is the question of reliable feeding of Grendel's short, fat case in a belt-fed machine gun. The SPC has a configuration much like that of cartridges that are proven to work.

Of course, the most critical aspect of a Grendel LMG is that it requires we ASS-U-ME viable disintegrating links can be designed. In contrast, no such assumption is necessary with the SPC case, which has dimensions compatible with the existing link design.

If testing (not just theory) were to prove 6.5mm bullets are truly better than 6.8mm projos, then IMO the prudent course of action would be to go with 6.5 SPC.