My big gas gun is a KAC pattern and I understand it is one of three common patterns; the other two being DPMS and Armalite. I know there are different mags and upper/lower dimensions. What is the difference (if any) in barrel extensions and gas tubes between DPMS and KAC? I am asking because I won a certificate for a CRA/BHW barrel at the last match and they only do DPMS pattern and is like to build a second upper in 6.5 Creed.