I saw a rifle in John Feamster's book that I am considering building. It has a 20" bull barrel with a carbine length gas system and handguard. Mostly it just looks cool to me, but I want to make sure before I invest that this design will cycle reliably, and to avoid any inherent complications. Compass Lake assures me that it is reliable. What about rapid fire? Am I better off just going with an M4 barrel? I already have two; a carbine with a 16" SUM, and one with a 24" bull. I guess you just can't have too many. Thanks for any info.