SouthPaw, I'm assuming the magazine in your dads rifle is just for decoration? Or are you managing to use 5.56 mags with the Grendel?
Ok, so I took a 6.5 elander mag and a 5.56 usgi mag apart. Identical. Followers are the same, grooves in the sheet metal are the same, curve of the mags are the same, metal thicknesses the same. Only difference I can find is gloss black, vs flat black paint.