It's not as difficult as you make it sound. For example if you're in possession of pmags, it's very easy to prove those are post ban magazines since the company didn't exist in 1994. If you're in possession of magazines for a gun that wasn't manufactured before 94, that's also easy to prove their postpan. Has more time passes between 94 and today it gets easier and easier for the government to do that. This year will mark 30 years since that assault weapons ban was enacted. Most of the new firearms that people want to own, absent Glocks, began being manufactured after 1994. It's very easy for the state in this instance to prove any magazine you have that is standard "high" capacity is post ban.