The only reason to buy that kind of weapon is (1) if you are a terrorist, or (2) if you are insane, either paranoid or psychopath or both. There is no legitimate, Constitutional excuse to own any semi-automatic weapon, any more than there is to own a bazooka. Neither was in the minds of the drafters of the Second Amendment.
There is no legitimate defense of continuing to allow such gun purchases by terrorists and the insane, except that the person in favor of those sales is either a traitor or mentally incompetent. I stopped donating to the ACLU decades ago – their fundraising scam sham surveys are just too dishonest.
Yes, the no fly list is a problem. Adding this to it will help prompt the reforms needed there. Being able to travel is a legitimate right. Owning a semi-automatic weapon is not.