As far as as I can tell, the biggest difference is the follower. The .223/5.55mm follower has a shallower rib than the Grendel follower. Another difference could be the first internal ribs in the mag body that guide the bullets into the mag. The space between the right & left ribs appears to be a bit wider in the Grendel mag than the .223 mag. The larger diameter bullets in the Grendel cartridges would surely necessitate this.
I measured some of my mags: An aluminum, 20 round G.I. 5.56mm mag was .570" between the ribs. A commercial, stainless steel 30 round .223 mag was .600" between the ribs. Two of my stainless steel Grendel mags were .640" (10 rounder) & .644" (25 rounder).
I measured some of my mags: An aluminum, 20 round G.I. 5.56mm mag was .570" between the ribs. A commercial, stainless steel 30 round .223 mag was .600" between the ribs. Two of my stainless steel Grendel mags were .640" (10 rounder) & .644" (25 rounder).
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