I built my MK12 with the BA SPR barrel in 6mm ARC. this was before Geissele had 6arc barrels available. i freaking love it. im about to order an AEM5-30 in FDE to complete the rifle. for now i am running a sandmanK on the end.Honestly, the 18" Geissele 6 ARC checks a lot of boxes in terms of function.