Murder is against the law. If criminals will violate that law, commandment, and moral issue then what makes politicians think that criminals will respect laws that outlaw a murder weapon?
The crooks can get their guns (legally even, if they have not committed a crime) elsewhere and bring them to town.
It won't stop or stem crime. It will just trample on peoples right to self-protection and make them "criminals" under the new statute.
All one has to do is look at gangs to know that anyone who wants a weapon can have one.
Regardless of whether you have been a felon or not you can get a weapon at any time or place.
There is always someone willing to provide a gun(market forces at work).
Maybe that's the point. More laws, more criminals, more jails, more cops, bigger government, more taxes, more perks and bennies for politicians. Will it ever end? Nope. The nature of government is to create a vacuum where none exists and then fill it with subjects.