Hey guys, I know this is a repeat topic but could anyone supply me with the best diameter for the gas port on an 11.5 inch barrel?

 Thanks!

 moon

moon

These are good starting points.  A few thousandths is a significant change.

barrel/gas port
20/0.093
16/0.063
14.5/0.067
11.5/0.076
10.5/0.089

I'm interested in where these numbers came from because they happen to be ass-backwards from what they should be - i.e. the shorter barrels should actually have larger gas ports, not smaller.  Of course now Colt has started using .092" for all barrels, regardless of length.  Read all about it [url=http://www.ar15.com/forums/topic.html?b=3&f=4&t=176929]here[/url].

FWIW

14.5/0.062
10.5/0.070

[url]http://www.dtic.mil/ndia/2003smallarms/john.ppt[/url]

Homo

The gas pressure at the port depends on 2 things.  The available pressure decreases as the gas port is moved further from the chamber.  The amount of time the pressure remains at the gas port depends on the length of barrel after the port.

The 10.5, 11.5, 14.5, and 16" barrels have the gas port at a higher pressure point than the 20" barrel.  The 14.5" barrel has the same amount of barrel after the port as the 20".  Thus, the 14.5" barrel should have a smaller hole.  

.092" for a 14.5" barrel is too much.  The extractor could start ripping part of the rim off.  This would be even worse with a colapsible stock, lighter buffer, and shorter receiver extension.

 Thank you very much for your time, guys.  Oldguy, thanks a bunch for your input and exlanations of the dia/barrel length relationship.

There's a portion of the FAQ that covers this
[url]http://ar15.com/forums/announcement.html?b=3&f=66&id=193[/url]

Some collected sizes FWTW,

11.5" barrel, .625: dia., 3.850" from muzzle;
       Port sizes from .081" to .089"
11.5" barrel, .750" dia., 3.850" from muzzle
       Port sizes from .086" to .094"

14" barrel, .625" dia., 8.375" from muzzle;
       Port sizes from .063" to .078"
14" barrel, .750 dia., 8.375" from muzzle;
       Port sizes from .070" to .086"

16" barrel, .625" dia., 8.375" from muzzle;
       Port sizes from .063" to 078"
16" barrel, .750" dia., 8.375" from muzzle;
       Port sizes from .070" to .086"

20" barrel, .625" dia., 8.375" from muzzle;
       Port sizes from .086" to .093"
20" barrel, .750" dia., 6.875" from muzzle;
       Port sizes from .093" to .096"
24" barrel, .825" dia.,
       Port size .089"

I have measured some of Colts 16" barrels in both diameters, and Colt uses two diameters of gas ports, .063 or .070" for the .625" barrel and .075" for the .750" barrel.

Barrel Length Gas Port
20"                .092
16"                .062
14.5" M4        .062
11.5" commercial .074
11.5" XM177   .067
11.5" carbine .072
LMG             .104
M16A2 20" .092+ .001
14"            .062+ .001
Commando 10.5" .081+ .001
LMG          .106+ .001
R6511 (222) 20" .0995+ .001
7.62x39 20" .125+.001

Thanks again to everyone for all of the great info and advice.  I am soon to be completing my Colt collection with a preban CAR 15 so I have an early, forged SGW gun that I want to completely retro fit once I get the Colt.  It's a good shooter ( the SGW ) but well used and, of course, no where NEAR being up to par with Colt, Armalite or Bushy. It's the only "cheap" gun that I own. Still, for it's terrible, slick, black finish, shot out barrel and lousy upper tolerences, it's a trooper...I just can't seem to part with it. I seem to feel a mixture of pity and love for it. I have been thinking of a slightly updated Colt Commando project...this is the perfect gun for it...adding something fun to my collection and bringing new life to an ugly duckly.

 A swan she shall be, boys, thanks again.