A 50 caliber rifle is legal and one has never been used to commit a crime. I think it has more to do with the cost of the weapon than anything.
Yea the cost aspect makes sense but that can be implemented to other firearms...make AR15 and other .223/5.56 so expensive to obtain that people stop using them.
Of course, you can do that with any weapon i guess and then people would just make their own. It's not like it's especially difficult to make a boomboom stick.