I am slowly getting the goodies together for my son's ar 15. I just received the upper from MidState firearms and all I can say they are fast-prompt! My question is on this buffer tube kit, do I want mil spec or commercial and what is the differance? I handled one of these complete guns for the first time about 3 month ago and I am ready to hock off all my levers and bolts for ar's(well almost). I want one in 6.8spc. I have done alot of research and still don't know much but my neighbor is a retired military man and he said he would show me how to assemble it so that should be better then me learning by myself. The upper we got is in 223 wyde.