What's the difference between the 6mm Grendel and the 6mm ARC?

[kmon]

Shoulder angle the same but the case body (head space) and overall case length are both shortened .03 inches for better use with Hornady 100+ gr bullets. Yup that is the difference in 6mmAR and 6mmARC. so basically about .4 grains less powder in the ARC but length is better for the heavier bullets