Uzi
.308 Win
I too came here to point out that the interpretation most of you are using is incorrect regarding ex post facto. They cannot criminalize an act already completed and then punish you for it having been completed while it was legal. They can change a law though, any law, to criminalize future acts that were previously legal.
In this example, you cannot be punished for having owned a magazine in 2011 that was legal then, but you can be punished now for owning the same magazine which was subsequently criminalized even though it was purchased legally because you are not being punished for the time period in which it was legal.