What makes you think they wouldn't just exempt themselves from the law, the same way they do with most other laws?
It’s not a matter of the law. It’s a matter of free enterprise; whether or not companies are willing to sell to them or not.
Gun companies should adopt a policy of not selling or servicing municipalities within gun hostile states that pass ridiculous laws that increase manufacturing costs and eliminate markets by making their products illegal.
When the withdrawal of gun makers from these markets actually started making things difficult for law enforcement to get the “tools” they think they need, they’d ask the legislatures to do something. And, it’s not like a government in NY can pass a law that a gun manufacturer in another state has to do business with them.
In other words, all gun manufacturers should give the Barrett treatment to those states who try to destroy their business: http://www.freerepublic.com/focus/news/836082/posts
I’d be happy starting with NY, CA, and IL.