Which is what I dont understand.. why didnt the Aurora and SH shooters use machine guns? Its so easy to get them.
It's NOT easy to get them - they're horribly expensive, and there's a long, involved application process.
The reason that machine guns are used so rarely in crimes has very little to do with the laws governing them - it's just because there are so few of them. There are nearly 300 million firearms in the U.S., and I believe that when the machine gun registry was closed in 1986, there were only something like 175,000 guns on it that could legally be bought and sold between private individuals. A large percentage of those rarely see the outside of a safe, so they're not likely to end up at a crime scene.