Green Mountain bbls
These guys do ML bbls and as I was shopping AR bbls for my shorty pewpew AR, I came across a M4 bbl assembly for 200 bucks. From what I found, I couldnt build one for that, maybe close if I hit sales, so I bought it. http://www.gmriflebarrel.com/gm-m51-m4-barrel-assembly/
It's boring, and your lack of creativity knows no bounds.