The source further reports that Mateen had purchased a Sig Sauer .223 caliber assault rifle at a firearms shop near his Florida home, St. Lucie Gun Sales, on June 4 and then a Glock 17 at the same store on June 5. Mateen had returned to the store a third time on June 9 to buy magazines for his weapons. The store is a federally licensed firearms dealer. Under law, the seller would have had to notify the Federal Bureau of Investigation of Mateen's purchase so that his name could be checked against the National Instant Criminal Background Check System, or NICS.I know that his name was on two FBI watch lists but currently that doesn't mean anything. He hadn't committed a crime so nothing could stop him from buying a gun. If they changed the law it would infringe on the rights of others. Being investigated or having been investigated should not be a crime.
However, an idea I was thinking about would be to allow gun shop owners to deny service to anyone they want to without running afoul of the ACLU. What this would mean is that they could conceivably discriminate against people with Muslim surnames and get away with it. This would be a special case only for gun shop owners and serve to make them a protected class of retailers.
The problem with this plan would be it stops law-abiding Muslims that simply want to defend themselves. That would strait-up infringe on their 2nd amendment rights. Also such a plan might have stopped Mateen or the San Bernadino killers but would not have stopped the Virginia Tech shooter, Sandyhook, or the Colorado shooter or the Boston Bombers because they all would dodge the ban for circumstances that were unique to their cases.
Another problem with this plan would be that Muslim gun shop owners might pop up to get around this idea if there was enough demand. In fact I could see deep-pocketed individuals like the Saudi's or Iran bankrolling gun shops that do not discriminate against Muslims. I could see ISIS starting a gun shop to funnel guns to would be Mateens. In fact I wonder if the FBI is investigating such a thing?
This plan also doesn't stop an Islamic terrorist from buying a gun at a gun show, online, or from a straw-purchase. I'm convinced Mateen would have did one of these things if that St. Lucie Gun Sales would have denied his purchase. So other than making it harder to buy the guns and ammo this kind of thing would have only slowed Mateen down. Only ideological change would have actually stopped it from happening. That would only happen in the Mosque or maybe if we destroyed ISIS once-and-for-all. This might unfortunately be the new normal.