To continue where I left off, the reason we have a standing military is thanks to Alexander Hamilton. I unified the militia so that the United States as a whole could conduct war without having to delegate that power to the states. The law was passed before we got into the War of 1812 and there is no reason to arguing on about it. The Feds have the power and it will never be rescinded.
As for a gun ban? No. That won't work, but I think more laws pertaining to the sale of firearms would be the right course of action. In Australia, you have to apply for a license, which takes up to 28 days before your application to apply for one comes back, then you can get your license for a type of gun (handgun, rifle, etc), and then to own a gun you are required to state your reason for purchasing a weapon (self defense is not an acceptable reason there, a gun can be used in self defense, but its sole purpose cannot be to kill other people. Hunting, Sport, Collecting are all viable), a gun safe, and trigger lock.
There are additional laws as well, but those are the more important highlights. Tough again, Australian laws are usually tougher on everything, like DUIs for instance. Do that once and you are done. Texting while driving too.
Just making sure every firearm is registered, licensed, and locked up will stop things from happening. I do also believe we need to monitor ammunition sales as well, as Aurora wouldn't have happened if we didn't allow the sale of 6000 bullets to be sold to that maniac.