Of course they can. You can build a fully automatic weapon in your garage if you have the lathes and equipment. It's illegal as to modify to full auto without a registered sear and completely illegal to manufacture without a class II license. So nothing legislation can do to make it more illegal. When fully automatic weapons become a problem, we can talk about that.
The problem is people focus on the peripherals like how much ammo he had, how many guns he had, the bump stock, all this . He could have rammed a truck through that crowd and killed a ton just as easily. No one focuses on the truck or trying to make it harder to rent a van or truck.
If he had a suppressed rifle with subsonic rounds he could have killed people all night, here and there and still be on the loose.
I'm not in favor of work arounds for things like the NFA or 1986 ban on autos by having bump stocks. I'm not in favor of a lot of that's a side issue to the gun debate.
People always look for the post mortem "can't happen again" root cause analysis however sometimes people are just ing crazy or they hate others and decide to go out in a blaze. It sucks, but that's a risk you take when you pack yourself into a crowd of 20K, outside in a compromised position, lit up like a target. You don't know you're taking that risk and in an ideal world it wouldn't be a risk to have a good time but even in church you can be gunned down if you drew a short straw that day. The best you can do is have the ability to send lead back the other way if at all possible, maybe stop the attack, maybe not. Maybe even make it worse, but these people won't attack in situations that don't favor a high body count.