Quoted:
If you think you're going to change up your sight or handguard arrangement, then 16" is the way to go.  I went 16" just to not hassle with a pinned flash hider.

Quoted:
Well I personally prefer the 14.5" in a carbine. All six of mine are 14.5" with permed FHs or SBRed. I find them much handier and frankly, that 16" barrel with the M203 cut looks kinda gawky. My opinion only of course.

Quoted:
http://www.03designgroup.com/photo/carbine-vs-midlength/icon-carbine-vs-midlength.jpg
03designgroup | <a href="http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system" target="_blank">Carbine vs. Mid-Length Gas System on a 16" Barrel http://demigodllc.com/icon/extwh3.png</a>


Keep in mind that the
-14.5" with carbine length gas system  
-16" with mid-length gas system
-18" with intermediate gas system
-20" with rifle length gas system have approximately the same dwell time (distance from the gas port to the end of the barrel).


Click on this link:  http://www.bushmaster.com/anatomy_bushmaster.asp - then click on "SPLIT" at the bottom of the screen.  Then watch the cycle of operation.  Note where the gas port (gas hole) is located.  Watch as the bullet travels down the barrel and past the gas port.  Think about the information I posted above.  If the barrel were longer, it will result in more gas being pumped into the gas tube and into the action of the carbine.  More gas equals more force, more force equals a higher recoil impulse.  

Kind of hard to explain on the internet, much easier to show someone face to face.


Hope this makes sense.

Quoted:
14.7" would be better. YMMV

Quoted:
14.5" for carbine gas system, and 16" for midlength. Since you want a carbine gas system, I voted for 14.5.

Quoted:
Been considering this question myself, however it seems to be a better option to go with a 16" mid length, and not have to pin a FH. Gives you more options later.