There are enough people doing this, the trying to keep track of them would be functionally useless. When Obama came into office there was a wholesale panic in the shooting community that the bulk buying created serous shortages. Even if the sales were tracked, what would be done about the data? Go ask each one if they're planning a massacre. And even trying to set up a system would kick a political hornets nest. Even if that could be done, trying to take action agains someone for just purchasing what is essentially a legal commodity would create even a bigger firestorm. Gun control has become politically radioactive for a reason.
Whatever you think about pot smoking and illegal immigration, these people have actually done something illegal. What you're talking about is trying to predict who will do something illegal. That's a big step.
Sorry that ship has saled. With the SCOTUS Heller and McDonald decisions, the question of whether there's an individual right to gun ownership (and not just your small pistol) has been decided, and not in the way you'd like.