I wish Mr. Tresmond well but don't believe the bill passed is an ex post facto law. As I understand it, the law would grandfather in ten round magazines already in a gun owner's possession but limit the owner from loading more than seven rounds in the magazine. A law violates the Ex Post Facto Clause only when it criminalizes past action. It is the future act of loading or possessing a magazine with more than ten rounds in it that is being criminalized.
The Second Amendment itself provides a better chance of success than an ex post facto claim.