Regulate not the sale of guns themselves, but the purchase of ammunition, along with large capacity magazines.

Regulate the purchase of ammunition to stores only. First, remove the ability to purchase high capacity magazines from the internet. Furthermore remove the ability to purchase rounds from the internet. By doing this you remove the ability to maintain anonymity while purchasing ammunition. Secondly, require that each purchase of ammunition, be registered with the FBI, Just as the purchase of firearms is, the FBI will be able to compile a list of individuals attempting to purchase high volumes of ammunition.