who makes the best barrel extension

They are all the same ecept for Les Baer and Armalite. As you will not get one from Armalite right now give LEs a call but as he makes them and all his stuff inhouse they are not cheap.

Its very easy to file them especially if your receiver already has them as you are only matching them up. All you have to do is cut them so that you cut down till the end of the extensaion where it meets the receiver ramps is the same size ot very slightloy larger will not hurt. Use a round fiuel or dremel witha conical gridning stone attached. Take a littel them seat the barrel in the reciever and do some more until it matches up. Once you get the first one done the second will be done in 1/4 of the time. CA Kontonics did all of their recievers in house as they used STAG. AFter you get done polish it up with metal polish and a buffer whatever to aa nice mirro shine and you are good to go. 15-30 mins tops if you haven't done it before.