From The Volokh Conspiracy:

The lawsuit alleged that, “It is practically impossible, both for those subject to the law and for those who must enforce it, to determine whether any of the thousands of different types of ammunition cartridges that can be used in handguns are actually ‘principally for use in’ or used more often in, a handgun. The proportional usage of any given cartridge is impossible to determine, and in any event changes with market demands.”