6.8 SPC Magazines and Chamberings for AR2

6.8 SPC didn't really die out... yet. Another funny chamberings that is just fairly not so known.

The chambering now exists in ModulAR2 and also a set of magazines with PMags and STANAGs.

The magazines aren't accurate to what the 6.8 SPC uses since the rounds are a bit chunkier and tapered, but unless we got proper DURAMags, we'll throw them into the STANAG and Pmags for now!