How do you measure a barrel?  Do you include the barrel extension?  What is the actual length of a 20, 14.5, and 11.5 inch barrel excluding the flash hider and barrel extension?  

Also, does anyone know the actual diameter of the A2 barrel forward and aft of the front sight base.

Thanks in advance.

Fronsight

IIRC, the BATF will take a wooden dowel rod and drop it down the barrel, mark it at the muzzle, extract it and use a tape to measure the barrel length.

Mike

By the way, the bolt is closed on an empty chamber when the wodden dowel is inseted for barrle length measurement purposes.
A real M4 carbine barrel (14.5") measures about 16" using this method, because it includes the flash suppressor.
M16A2 rifle barrel is 20" measured this way WITHOUT the flash suppressor installed.  So in the military, the flash suppressor length is in addition to the barrel length.
M16A2 barrel diameter behind the front sight tower is about .840".  The diameter "under" the sight tower is .750".  Forward of the sight tower measures about .725.
Cold Blue sends...

Thanks for the info.  So if i took a wooden dowel and measured the barrel on my full length Colt AR15A2 with the bolt closed and the flash suppressor removed I would get exactly 20 inches?  The reason I'm asking is because my barrel seems to measure a few hundreds of an inch more than the specified 20 inches.

Its the same for my supposedly 14.5 inch barrel with the Y-comp removed.  It measures more than 14.5 inches.  Hope someone can help me clear this up.    

Coldblue,  the measurements you gave me is for the HBAR barrel, right?  I always thought that the diameter of the standard M16A2 barrel behind the frontsight (under the handguards)is the same as the A1 barrels, .625 or something?

Again, thanks in advance.

Frontsight

Barrel length is measured down the barrel to the face of a cocked bolt.  There is no specification to use a wooden, titanium, or kevlar rod to measure this distance .

You really don't think a 20" barrel is 20.000000000" long do you?

-- Chuck

.625...no way.  When we increased the end diameter of the A2 barrel in 1980-1983, we brought the front end's larger (read: stiffer) diameter to about 1/2" behind where the handguard cap is "trapped" between a shoulder on the barrel and the rear face of the gas block/front sight tower.
A1 Rifle and old XM177E2's carbine barrels were this smaller .625 diameter.  Colt 9mm barrels remain this smaller diameter as well.  Thyat is why our RIS is our recommended rail forend for the 9mm, vs the M4 RAs which needs the .840 diameter up front to stabilize properly.
Real Colt-H-Bar, as I remember, justs continues the .840 behing the front sight tower, fully to the chamber area (actually tapering to an even larger diameter. If you need one measured, I can pull one from the storeroom at work and measure it for you.
ColdBlue sends from home...

M16 barrel diameter up to the M16A2s introduction was .600" and porked up to .630" with the A2.  At the smallest points.

Diameter under the front sights is .625" and .730" respectively.

So called HBAR barrels go as big as .75" under the sight and can be close to 1" in diameter.

-- Chuck

Appreciate the info guys, thanks again.

Quoted: A real M4 carbine barrel (14.5") measures about 16" using this method, because it includes the flash suppressor.

A quick note:  The Flash Supressor (or muzzle brake) is ONLY included in the measurement if its PERMANENTLY attached.  If the muzzle device is removable it is removed prior to measurement.

BTW the A2 Flash Supressor (used on both M4s and M16A2s) adds about 1" to the length.  So you can't permanently attach it to a M4 to make a non-SBR (15.5" is close - but not close enough).  That is why you see lots of civilians pemanently attaching the longer Phantom or Vortex to their M4 barrels.

Also...  If you have a 22 LR upper that uses a barrel extension instead of the Ciener conversion 223 adapter, you can get VERY close to 16-inches of barrel length with what would otherwise be a 14.5-inch bbl!  

Sure would make a slick little 22LR upper!

DISCLAIMER NOTE, I've read that it comes out to 16.1-inches BUT I've never actually measured one myself...  Has anybody here measured one???