I've got a ghk m4 and love it. I'm going to get the heavy buffer and spring which is meant to be one of the best things you can do. Skirmished it first time last Sunday and it was awesome. Didn't have any 0.20s when I put it on chrono so used 0.28s and it was 284. I've got the adjustable nozzle in now so I can easily adjust. Will order some 0.20s when I order my gas week after next.