You can ban things from here to kingdom come, but unfortunately criminals, particularly those who, like this man and terrorist groups, have access to money, will continue to obtain just what they need to do their dirty work. You can also criminalize the possession of a modified weapon like the Las Vegas shooter reportedly had, but since he apparently gave absolutely no indication he had a modified weapon, had a mental problem, or had murderous tendencies until he just let loose one day, I doubt a law would have stopped him.
If we are going to talk about gun regulation in the USA, it should be aimed more at reducing the mundane gun violence in places like Chicago that results in deaths daily and over and over. The regulations in that case aren’t working very well and the death toll over time dwarfs the mass shooting incidents. Gun laws need to be enforced and that means a lot more stop and frisk and sending people to prison, both of which are controversial as most of the people affected will not be the lone wolf mass shooters and terrorists, but will be poor minority men.