It uses AICS short action pattern mags. You can get AICS mags in different calibers such as .223 or 308/6.5 but the lockup dimensions are the same.
Because of the action size and spacers in many 223 AICS type mags Magpul does not guarantee they will function with the smaller case sizes. That's why they are making the Stanag/AR Hunter stock also. It should be more reliable with smaller cartridges.