This may be a silly thought...
I was browsing the net a bit and was tossing the idea of trying to build a 300 BLK upper and a rock river lower. i saw a 9.5 PDS on PWS website and rock river makes a neat folding stock capable lower. I dont know if they sell it seperatly though. I would probably cut the barrel down a bit so the addition of a suppressor doesnt make the overall length too long.
then again i keep eye balling the novenski 8" 300 BLK rifles already done and ready to go.