In spite of its imperfections, the Heller decision should have made it clear, for those who hadn't already known, that the 2nd Amendment is not about hunting: the right is to keep and bear ARMS. This makes all sporting purpose language in any gun control law, including the '68 GCA, ILLEGAL. Getting that language removed is the critical path in any effort to defend the right to keep and bear arms. It would do much to solve the current problem this thread is about and other problems too. Anything we do that does not address that root problem will be at best a bandaid.
The Heller decision makes much reference to arms being useful to a militia that would allow the people to defend themselves from a tyrannical government. This is an obvious purpose of the right even without their mention of it. If the American people ever do have to defend themselves against such a government, or an entity like the UN for example, armor-piercing bullets would be an important part of their arsenal. If the purpose (or a purpose) of the right is to allow the people to have the means to defend themselves against a government force, a law that has a stated purpose of keeping American citizens from having that ability ought to be illegal. They can't legally say that law-abiding citizens can't have a certain type of bullet by virtue of that bullet giving the people the ability to fight government agents, i.e., the police in this case.
Of course there's the way it ought to be and there's the way it is. Parts of the Heller decision are badly written and allow usurpations that should be illegal. And many Americans are more than willing to give up parts of their freedom that they have been led to believe don't matter to them. But those who seek to defend our Second Amendment rights should not give in to such things. If there is a right of the people to keep and bear arms, and the reason for that right is that armed people can better resist tyranny, then weapons suited to that purpose should be available to us.
There has to be such a thing as a reasonable restriction on a Constitutional right, but what is being done these days in the name of gun control is very far onto the unreasonable side of any realistic definition of what is reasonable. And laws based on the cop killer bullet lie are a case in point.