Grendel as a Universal Infantry Cartridge

[stanc]

My thinking on this subject has been gradually evolving. I'm now inclined to think that while 6.8x46 may be about right, the case diameter should be wider than the 6.8x43, for two reasons: the need to accommodate significantly longer copper+steel bullets, especially tracers, which will occupy more of the case capacity; and the desirability of allowing for the future use of part-polymer cases, which may have thicker walls than metal ones, thereby further reducing propellant capacity.

Okay, to meet all of those needs it looks like the .445" diameter case of the 7x46 UIAC is what will be required. (Possibly a .430" case could do the job, but I don't know of any viable parent case with that diameter.)

I think it is essential for the success of the concept that soldiers using the new round in combat must not notice any shortfall in comparison with the 7.62mm, especially in long-range hit probability, otherwise they will still insist on keeping the 7.62mm. If the new round can provide this while achieving significant reductions in weight and recoil, then I think it could kill off the 5.56mm, given that many soldiers have said that they would rather carry 7.62mm weapons than 5.56mm.

The available info on the 7mm UIAC indicates that trajectory and wind drift are comparable to, if not better than, that of 7.62mm, which seems to address your hit probability concern.

Felt recoil of 7mm UIAC ought to be noticeably less than 7.62mm, and my calculations show a 25% weight reduction relative to 7.62mm (which I'd rate as significant).