Craig is one my senators. I already wrote him on the subject. In plain words, I told him that if his legislation passes with all these anti-gun amendments, he should polish his resume. Idaho needs a conservative Republican Senator who won't compromise on 2nd amendment issues.
Even if it where, that wouldn't stop them. However that would not an ex post facto law, unless they punished for possesion in the past. If you could turn it in after the law banning possesion was passed or be punished for not turning it in, then that would pass the ex post facto test. An an ex post facto law is one that criminalizes something you did in the past, not something you did in the past AND continue doing after the law is passed.