And there are many legitimate sporting applications for 30 round magazines.
Here's one; 3 gun competition.
Don't forget the "President's Hundred Match" put on by the CMP, per CMP rules you must use either the 20 or 30 round magazine. Same with EIC (Leg Matches).
Not to mention you have to use the US Service rifle or Civilian Equivalent.
That's PRESIDENTS MATCH, not sure this president knows about it.