Who decides? Given that Obama and the DOJ considers veterans, "bitter-clingers," and outspoken conservatives and libertarians as either terrorists or potential terrorists, Trump really needs to re-think that position.
Trump is wrong on this issue on so many levels.
First, it cannot be emphasized often enough that gun control laws do not prevent any terrorists or criminals from buying guns. Gun control laws only prevent people who give a damn about complying with the law from buying guns. It is no different from claiming that passing a new federal murder crime will prevent criminals and terrorists from committing murder.
Gun control laws give criminals and terrorists the peace of mind that their victims will not shoot back. All "gun free zones" really are are "defense free zones." Putting up a "no guns allowed" sign is no different from putting up an "unarmed victims here, shoot away" sign.
The Democrats have gone from wanting to ban anyone on the no fly list from buying guns to wanting to ban anyone who they think should have been on the no fly list from buying guns. Either way, you are talking about a completely subjective list of people that someone in the government decided to include on a list. We don't know why anyone is on the list, since it is a secret. All we know for certain is that if you ban anyone on the secret list from buying guns, the secret list will suddenly include a hell of a lot more names.
Step one, ban anyone on the no fly list from buying guns. Step two, put everyone on the no fly list.