Associating weapon type alone with murder rates is not sufficiently indicative of the true cause. There are so many other factors that come into play.
What matters most is what we have at hand today, and that is assault weapons of unparalleled destructive power at affordable prices can be freely purchased by anybody over the age of 18. Nobody needs to own weapons of that magnitude today, analogous to the ownership of a howitzer or missile launcher. These are military grade weapons designed for killing many individuals in a very short period of time.
The people who want to own those weapons just don't want to relinquish the visceral joy they get from using them, just the way some people love to own 500hp street legal cars--all of that horsepower at your fingertips. It's reckless and irresponsible... but no, those against limitation will simply cite "freedom" and ignore "responsibility".