Hello to all! Can someone out there tell me the correct size of the gas port in the barrel for a standard 20" barrel?? A Carbine type 16" with full length handguards??  And a 16" M4 type barrel....  Do the different manufactures have different preferances? Or are they uniform between different makes? Thanks in advance! Till...later...GT

GT
There are variations.  It is not an exact science.

good place to start
20" 0.093" Bushmaster and Colt
16" 0.063" Bushmaster Rep
14.5" 0.067" Bushmaster Rep
11.5" 0.076" old guy
10.5" 0.089"

Check for a rough or sticky chamber as well as a gas leak before making permanent changes to the gas port.

Thanks old guy! What about the carbine version of the 20" barrel? (barrel is approx. 17" long, with full length handguards, front sight base near end of barrel) Is it the same as a 20" barrel port??  Can the port be drilled out on a chrome bore barrel? Where are gas leaks most prevelent? Thanks in advance for the info. great forum! Till...later...GT

I don't know anything about a 17" barrel.  If the gas port is at the same position as the 20" barrel the gas port would need to be larger to compensate for the shorter port to muzzle distance.

One way to find a gas leak problem is to chamber an empty and blow in the muzzle.  The gas flow should be barely perceptable.

Some like to say the gap in the gas rings should not be lined up.  I think only marginal rifles would be affected.  I've never heard anyone claim this fixed anything.  

A common gas leak problem is loose or broken carrier key screws.  They are staked so you need to check them with an allen wrench.

Hi oldguy! Thanks for the gas leak test info... sounds like a simple and effective test! The carbine barrel that I am refering to was introduced, I believe, in Viet Nam. I would assume for helicopter crews and such..   Bushmaster reintroduced this same barrel in their original dissapator barrel (sp?) prior to going to the same type port as a regular 16" barrel...  I see the DPMS catalogue had the same offering in an FN barrel, but it has been discontinued.... I had a early Bushmaster barrel of this type, but felt it was too heavy for my liking, so I have been looking for a military version of this barrel for some time, finally found one... but not sure if it is up to spec... or for that matter, if it is even original..  but, they do exist! I am going to clean and polish the chamber this week end... I think that it could be the problem, as has been suggested here on this forum... Thanks for the neat test suggestion, and best of luck to ya! til....later...GT

Years ago some 16" barrels had oversized gas ports, drilled the same as 11.5" military barrels.  Also, anytime there was a malfunction, first thing some guys would do is drill out the gas port.  Too much pressure is as bad as not enough.  The whole gas system of the AR is a balancing act, it has to be just right, just like Goldilocks' porridge.