Machine guns, generally speaking, aren't what we're using to kill each other. If you want to buy one legally, which is difficult to do given restricted supply, you will need permission of local law enforcement, pass a more detailed federal background check, pay some higher taxes, and from what I understand agree to more intrusive federal checks on your usage of the gun, whether you still have it and so on, all without warrant. You also can't sell it or transfer it without their permission. The people that have the time, patience and resources to do all that aren't for the most part in either street or organized crime.

Semi-automatics are a different animal of course but given the sheer number of guns, the right to keep and bear arms and the cultural place that guns hold, I don't think there is any support for revoking the 2nd amendment, which is what it would take to outlaw the ownership of semi-automatic weapons.

There ought to be more focus across all levels of government on identifying how criminals get their guns and stopping that flow. I think that's something most people can agree with.