Robinson Arms confirmed that with a CTR stock, you can use either use the military spec version or the commerical version so long as you get the corresponding stock tube. My question is what else other than the M4 adapter from Robinson, the stock tube and CTR stock, do you need to attach this to the XCR? I have heard of castle nuts, various types of screws mentioned. Does anyone know for sure what misc. parts you need and do they typically come with the purchase of a stock tube. Also, to my knowledge Robinson does not sell stock tubes.