what is unconstitutional about limiting the types of guns that are allowed?
We already have a very extensive set of state and federal laws that dictate what firearms are legal for a civilian to own, under what circumstances a person can loose their 2A rights, how to store and transport, and even a permit/licencing process in some places for things like automatic weapons, suppressors, short barreled weapons and concealed carry. If you want to include additional gun regulations or limits, those laws have to stand the test of constitutionality just like any other new law.
That means you can't make what is currently legal and in common use suddenly illegal in the hopes of casting that wide net to stop a tiny few individuals who misuse whatever you are trying to ban. At least that's exactly what the judge said in California when he ruled their 10-round magazine limit unconstitutional.
I've repeatedly explained this but the same argument "Well, you can't own a nuke. Duh!" keeps being offered up as some kind of wisdom. It's not.