Well, I'll let them speak for themselves. If I understand the argument correctly it's based on the 10th Amendment:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
The question is whether the 2nd Amendment's mandate to not infringe is covered by "nor prohibited by it to the States." ("It" here meaning the Constitution, of course). In my opinion it is so covered. But I'm not a lawyer.
Paulsen, will you "speak for them --"? Can you explain why an individual right is not "-- prohibited by it to the States." ("It" here meaning the Constitution, of course)?
An individual RKBA is "found" in the 9th amendment, if you will. But the federal government doesn't protect that right (the collective RKBA as part of a militia is protected from federal infringement, yes). So who does?
Each state, in their respective state constitutions, defines and protects the individual RKBA. If a state does not protect your right to concealed carry, for example, you have nowhere to turn.