There are a plethora of laws concerning use and possession of drugs. Until the 20th century, it wasn't illegal to use cocaine or opium. However, Congress used the commerce clause to regulate and control drugs of many types, effectively removing them from private or state control.
If you were a drug addict on April 3, your addiction would soon use up your supply, and whether you had drugs on April 4 would become a moot point, since the April 3 stash would be used up very quickly. After April 3, any procurement resulting in possession and/or usage would certainly result in prosecution. There is also the caveat, that some probable cause of illegal behavior would have to be known before anyone can be charged with anything. Also, no one knows how long you had the illegal drugs, since there's generally no manufacturing tag on marijuana, heroin, or crack cocaine.
Weapons manufacturers comply with a long list of Federal and state laws, which is one reason why there are fewer today than thirty years ago. The courts have generally been on the side of the manufacturer here. They produce a legal product, which for the most part is used for legal purposes. Courts have long taken a dim view of holding manufacturers responsible for legal products used in a legal manner. That's also why you can't be held liable if a person steals your guns and kills someone with it provided you report the theft(s). Courts have even held that you can't be held liable even if you don't report the theft.
Bills of attainder punish a person, or group of persons, by making something illegal, and bypassing the court system. If Congress and the President want to ban and seize a particular type of weapon, then they can certainly pass a law attempting to do so. However, a law suit would be filed in Federal district court before the ink has dried on the paper.
Congress passed a law concerning the AIG bonuses. They knew when they passed it, that it was a bill of attainder and an ex post facto law, since they knew and approved of the bonuses well in advance of March 2009. Their only intent was to pass a "we care, and we're mad as hell" law, and kick the can into the court system. They've targeted a specific group here, and courts have always held that this type of law was a bill of attainder.