I just bought a Smith M&P piston AR15 in .223. I want to buy a Rock River 6.8 SPC upper to go on this gun for the piggies. I was looking at 6.8 SPC loading data and some loads recommend using an SPC II chamber. Does this mean it can operate at higher pressures with reloads? I dont know what this Smith M&P is. I need to do some research to find out. Any info is greatly appreciated.