ive been thinking about for some time now on building an AR.however ive never owned one or anything.do i just go buy an upper then the lower with the parts kit and put it together?id like to get an upper in .223,.40s%w or 7.62x39.what kind of mags would i use if i were to get a .40sw or 7.62 upper?i seriously have no clue and any help would be great thank you.