Comparison Study

These three uppers are thematically similar.

The top barrel is 16" with a mid-length gas system, the middle barrel is a 16" with a carbine gas system and the bottom is a 14.5" with a carbine gas system.

If you are building an AR carbine without paying $200 for a tax stamp, you'll have to use one of top two options.

Much of it is a matter of cosmetics.  There's not much difference in function.  The mid-length is actually what I think is ideal for a 16" barrel for gas pressure and dwell time.  Superior to the carbine system in most ways.  A carbine system with a 16" barrel has too much dwell time after the bullet passes the gas port so more gas is blown back into the bolt carrier and by extension into the action area.  14.5" is where Colt figured out you needed the length with the carbine system after several years of trying various fixes to make shorter barrels function properly.  Note that the 11.5" XM177E2 barrel has a 4.5" long "moderator" on it that brings the overall length to nearly the same as a 14.5" barrel would have; why not have 3" more barrel to get that extra bit of velocity?

It's not much velocity, but it does have an effect.  The Ammo-Oracle has a nifty table showing that effect.  This only applies to military issue ball ammo, some commercial hunting ammunition has superior wounding effects at lower velocities but would be banned from military use by the Hague Convention.

Distance to 2700 fps	20" Barrel	16" Barrel	14.5" Barrel	11.5" Barrel
M193	190-200m	140-150m	95-100m	40-45m
M855	140-150m	90-95m	45-50m	12-15m

Many people who have made XM177E2 clones have retained the 16" barrel and have a moderator that has a lot more threaded distance than the normal half inch.  This maintains the appearance of the shorter barrel with the functionality of a longer one.  My first attempt with Kaylee had something similar with a 5" "slip-over" flash-hider.

Again; much of this is more about cosmetics than it is about function.  I wanted the gun to look right.  In the above picture you can see that both the 16" barrels are too long in some respect as compared to the 14.5".  The mid-length has too much handguard (and has a triangular handguard cap).  The carbine length has too much barrel.  In the real world, it's hard to see the difference.

How hard?  Get a copy of the movie, "Platoon".  The carbine Tom Berenger's character, SSG Barnes, is carrying appears to be a Philippine R635P.  That has a 14.5" barrel.  However, in some scenes it appears to have a 16" barrel.  You don't really see it until you pause the movie to see some other detail and then notice that there's more barrel in front of the front sight than the M16A1 in the same shot.  I'll bet hardly anyone but gun-geeks like me even noticed that the actor got handed two different props.