Textron Systems NGSW Rifle and LMG

[stanc]

Caliber is relative to the external and terminal ballistic performance required, and thus is driven by the performance parameters set forth.

The point is to select the smallest caliber — in order to keep system overall bulk down — that will achieve the performance parameters.

Yes. Isn't that precisely what the Army did?

As far as a 6.8 NGSW goes, a 6.5 NGSW would have achieved the exact same performance, only with better external ballistics.

A dubious claim. IIRC, the Army reportedly tested a range of calibers, and determined that those smaller than 6.8mm failed to give the desired performance.

Plus, reducing caliber to 6.5mm would give a reduction in cartridge diameter of little more than one hundredth of an inch (0.01").

The difference in system bulk -- gun, mag, cartridge -- would be virtually nil, and magazine capacity would remain at 20 rounds.