No one should be against making it illegal for anyone under 21 buying a AR-15. You do not have to outlaw ALL guns. But you do not have to protect the "right" to buy semi-automatic weapons for teenagers.
There is a middle ground.
Chicago--on a weekly basis--has longer casualty list than at Robb Elementary. The solution is not gun control. It is perp control. Breaking the law with a gun, carrying a gun without a permit etc? Try and convict. Not catch and release.
But the NRA has a big problem on their hands this weekend. Background checks, red flag laws and age restrictions for AR-15 are (and should) be on the table.