I'm wanting to buy a magpul ACS stock. I have a 308 AP4 with a 6 position collapsable stock and I'm confused on whether I need to buy the commercial or mil spec diameter? I'm preety sure my research points to the commercial but I don't really know. Any ideas? and I dont see any reason why I cant use the old buffer, buffer tube, and recoil spring? http://www.midwayusa.com/viewProduct/?productNumber=912037