There will be no exemption in the proposed federal laws for privately owned arms owned by members of State Defense Forces. Even though an AR-15 would likely be more suitable for militia purposes envisioned by the 2A than a shotgun, if the AR-15 is banned you will be out of luck (unless the Supreme Court says otherwise, which is entirely dependent on how Justice Kennedy feels and if Pres Obama gets another pick before the case is heard).
The sort of law that would be effective from the state level would be something like the Firearms Freedom Act under consideration with teeth. It needs to add a clause like, "It shall be a felony for any South Carolina state employees to aid or abet federal enforcement of federal firearms laws" at a minimum. This would shut down all "fusion centers," professional courtesy, use of state personnel to enforce federal laws in "partnered" operations, etc.
One could set up an even stronger law with another clause that states, "It shall be a felony for any person to to aid or abet enforcement of federal firearms laws." Of course, using SLED agents to arrest ATF agents would not go over well in Washington DC and I can expect that there would be significant backlash. Even suspension of federal money would cause significant budget problems for SC.
Of course, enough states (3/4 of them) could also propose an Amendment to the US Constitution. For example, something like a modification to the 2A that states, "The individual right to keep and bear arms shall not be infringed, and any proposed restrictions shall be subject to strict scrutiny." Louisiana passed a "strict scrutiny" modification to their state Constitution this year and Alaska clarified the individual nature of the right back in 1994. Both serve as models for clarification of the US Constitution if necessary to prevent the right from being read out of existence by the courts and Congress.
