The Gun Control Act of 1968 is the law that made it a crime for felons to possess firearms.

It is a bad law for the obvious reason that once you allow one group's rights to be denied, you are on the slippery slope where the bar can be lowered until everyone is denied. And that has happened - the Lautenburg Ammendment, if you have a restraining order against you, etc.

But it is also unconstitutional because it is an ex post facto law. A convicted felon in 1967 was able to legally own firearms. It became a crime for him to do so in 1968. This is an increasing of criminal penalties after the fact. Ex post facto laws are unconstitutional.