I am going to go with a 20" barrel on the "build" I have just started.
Why?
The Military uses 18", but they are stepping up from 14.5" for a gain of 3.5". I already have a 16" Midlength Carbine, so for me to get at least as much of a gain in velocity as they do, I need to go to 20".
Besides, when I was in the Army, I had an M16A1 with a 20" barrel, so I'm used to it. The only AR15 I ever owned before my 16" Carbine, was also a 20" DCM gun for Highpower competition.
Hey MOLON, how long is the barrel of the gun in your post??? You only said "Other".

