There are a few facts here that are either unintentionally or intentionally misrepresented. First of all, the gunman did not use an AR-15. (Just for the record, even if he had an AR-15, an AR-15 fires one shot every time you pull the trigger–it is NOT an automatic weapon–it is just a scary-looking rifle.) He used a Sig Sauer MCX rifle. Similar, but again, not an automatic weapon. About the idea of taking guns away from people on the terrorist watch list or the no-fly list. Oddly enough, this is a really dangerous idea in relation to the U.S. Constitution. Both these lists (aside from their record of inaccuracy) are done without the person on them being aware of being on the list. The person placed on the list has to go through a lengthy process to get off of the list. This is an assumption of guilt and having to prove innocence. That is against the U.S. Constitution. In America we have the right to face our accusers, and we are considered innocent until proven guilty. Taking guns (or forbidding the purchase of guns) to a group of people who are not charged with any crime and have not been proven guilty is a really bad idea.
Just a note on the no-fly list–at one point Senator Ted Kennedy was delayed when flying home to Massachusetts from Washington, D.C. because somehow he (or someone with the same name) had been placed on the no-fly list. The no-fly list is not a list I have a lot of confidence in.