Like Ms. Publius said, it would not be ex post facto, because the law will be to “turn in your gun.” You won’t be committing a “crime” until you refuse to turn in your gun, because you would not be charged for formerly possessing the gun, but for not turning it in.
None of that matters anyway, because the answer will be “NO”.
“because the law will be to “turn in your gun.” “
No, the law will be “possession of”, not “turn in”. Besidesm “turn in” also then gets into other issues of being paid market value.