The US Specials Forces have made confirmed kills out to 5 or 600 yards (I can't remember which and I'm assuming that it had an Elcan SU-230 mounted on it). I have built the big brother, Block II M4A1. 14.5" barrel and M4A1 RIS II, KAC NT4 flash hider, MK12 gas block, etc. The MK18 will be my next build. Both versions are unreal. You won't have any regrets if you build/buy this carbine. I've handled a Daniel Defense MK18 at Wolverine Supplies and the quality of these rifles is amazing. Zero slop between the receivers. If the upper receiver from DD didn't have DD advertising on the side of it, didn't have a Magpul trigger gaurd and it actually had the MK12 gas block I'd buy it rather than build it.