Magpul claims the internal ribs of the mag would have be too thin and would weaken the magazine too much to work in a milspec lower. The LWRC lower has a wider than milspec magwell.
I just finished putting together a 6x6.8spc and have not shot or played around with any magazines yet, but I plan on trying pmags. I have modified 5.56 pmags to work with the wssm and the wssm case is fatter than the spc, so I don't see why a 5.56 pmag couldn't be made to work with the spc with some modifications to the follower and feed lips. Only problem I see is the limited OAL.
