Re: How come...?
I had a Harris Gunworks (back when he was using the MacMillan name) .308 that took M1A magazines. It worked but you had to tweak the mags, i.e., polish the feed lips and tune the springs. The installation of the hardware left a huge opening in the stock both side-to-side and front-to-back, potentially weakening the area behind the recoil lug. Twenty round magazines hang down from the rifle quite a ways making prone shooting a bit awkward. Five rounders don't extend far enough past the belly of the gun for you to manipulate them quickly. You'll also have to break position to rock the magazine in and out of the gun to reload.