After some range time on the KAC URX 3.1, my next build will be sporting one. I really liked the weight, length and overall feel a lot more than I thought I would. It is a URX so that means its not exactly drop in and ready to roll. You do need a specific URX Barrel Nut Wrench or a gunsmith who has one. From what I remember the wrench did used to cost a pretty penny but I believe they have dropped down a little.
http://i49.tinypic.com/oq93sg.jpg

