I have 2 ar-15's now 1 20"hbar + 1 16"hbar,both in .223.I am waiting on a new lower from bushmaster and was thinking of a .45 or 10mm upper,but only if i can get high cap mags.A few years ago i saw a ad for a .45 that used 30 rd thompson mags.Is there a upper out there that is good or should i just get another .223 upper?