Originally Posted by
gotm4
The carbine length gas system has been around since the Colt XM-177 in the mid 1960s.
It is my belief that since the carbine length gas system worked well on the short barreled XM-177s Colt probably didn't want to reinvent their own wheel and just used the carbine length system in the XM4 (in the 1980s) then later in the M4 and M4A1. The weaknesses of the carbine length gas system probably weren't realized until the M4s adoption when it was subjected to really heavy full-auto use by Ranger units etc.
The USMC never wanted the M4 to be a replacement for an M16A2. The M4 was just going to be the next carbine (used like the M1,M2 and M3 carbines had been) which is to be issued to commissioned officers, staff NCOs, tankers, MPs and others who would be typically only issued pistols. It was meant to give more power and range than a pistol and smaller and easier to carry around vs a full sized rifle. The original XM4 (A2 type upper) had 85% parts is common to the M16A2.
Whenever you have a choice in a 16" barrel the mid-length should be the 1st choice.