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 Colt gas port size?
jwmac  [Member]
2/26/2012 1:43:14 PM
Anybody know if Colt gas ports are generally a little smaller than other manufacturers?

Does it depend on the manufacture or is there a set standard size for gas ports? Is there mil-spec and commercial standards? 223 - 556 differences?
WI57  [Team Member]
2/26/2012 1:45:02 PM
When I was doing research for my SBR I was finding info that Colts gas ports were actually larger than other manufacturers.
Dano523  [Moderator]
2/26/2012 2:02:21 PM
Originally Posted By jwmac:
Anybody know if Colt gas ports are generally a little smaller than other manufacturers?

Does it depend on the manufacture or is there a set standard size for gas ports? Is there mil-spec and commercial standards? 223 - 556 differences?


In general, Colts have the larger cross-wall side military nato chambers, and have set the standards for barrel gas port sizes.

Now having said that, they do have barrels for suppression, and even for heaver grain bullets that will use slower burning powder, and in these specific barrels, will use a smaller gas port size since either the suppressor or slower burning powders will increase the pressure that the port will see, requiring it to be smaller than normal.
jwmac  [Member]
2/26/2012 3:29:41 PM
Originally Posted By Dano523:
Originally Posted By jwmac:
Anybody know if Colt gas ports are generally a little smaller than other manufacturers?

Does it depend on the manufacture or is there a set standard size for gas ports? Is there mil-spec and commercial standards? 223 - 556 differences?


In general, Colts have the larger cross-wall side military nato chambers, and have set the standards for barrel gas port sizes.

Now having said that, they do have barrels for suppression, and even for heaver grain bullets that will use slower burning powder, and in these specific barrels, will use a smaller gas port size since either the suppressor or slower burning powders will increase the pressure that the port will see, requiring it to be smaller than normal.


I'm guessing these barrels for suppression, etc would probably be marked as such? Or would the only way to know require removing the FSB and measuring the gas block?
Dano523  [Moderator]
2/26/2012 6:56:54 PM
Majority of them are 14.5" barrel, but more of a upper receiver destination then a barrel destination you would be looking for when they are ordered.

And no, it would be the barrel gas port size, and not the FSB size that would be smaller, since the gas passage channel on the FSB is the same for all the units ( comes down to either standard or F FSB, depending on the barrel it's used on).

Now to save you from having to type even more, once you remove the FSB, you can check the barrel gas port size to see if it is one of these barrels. Here are the standard ranges for gas port sizes on standard Nato barrels for M-193/ M855 ammo.

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"
Combat_Jack  [Team Member]
2/26/2012 7:35:08 PM
Dano the 14.5 and 16" M4 barrels are .063.
Dano523  [Moderator]
2/26/2012 10:39:06 PM
Originally Posted By Combat_Jack:
Dano the 14.5 and 16" M4 barrels are .063.



Yes, 99.9% of the time, but there was a lot of special order barrels for suppressed mk-262 that had undersized barrel gas ports that made its way into the civilian market a while back (not sure if these where over runs from the contract or the contract canceled and piece sold off instead).

Not sure who there where meant for, or why they ended up on the civilian market with out the gas ports just being increased before they where sold as standard barrels, but If I remember correctly, the gas port was in the lowerish .050'ish range on these barrels.

And for the record, there is no such thing as a mil-spec M-4 16" barrel. The single reason behind this, a standard bayonet will not work with a 16" barrel (ring of the it around the FS when it clips into place. instead of just the ring just dangling on the barrel meat instead when clipped into place).
jwmac  [Member]
2/26/2012 11:09:28 PM
Thanks for the good info guys. Sounds like .063 is most likley the size for a Colt 14.5" barrel unless it was made for special circumstance. Being that .063 is what Colt is using this seems to support the fact that Colt barrels (14.5 and 16" anyway) would indeed tend to run on the smaller end of the range for gas port size. I have read elsewear that some civilian manufactures tend to run slightly larger gas ports to allow for more reliable funtion with the common cheaper quality (lighter loaded) ammo out there. Sounds like the defense manufacturers such as Colt, LMT, etc. tend to run their gas ports on the smaller end of the range due to the higher quality ammo the weapons are more likely to run (xm193, m193, m855, etc.) which would run the weapon just fine.

Am I on the right track here? Can you guys confirm? Deny? Other info?
Combat_Jack  [Team Member]
2/26/2012 11:15:07 PM
LMT is .058 IIRC and some of the lower quality manufacturers really punch a big hole. Bushmaster in particular is known for it.
Dano523  [Moderator]
2/27/2012 11:31:02 AM
Originally Posted By jwmac:
Thanks for the good info guys. Sounds like .063 is most likley the size for a Colt 14.5" barrel unless it was made for special circumstance. Being that .063 is what Colt is using this seems to support the fact that Colt barrels (14.5 and 16" anyway) would indeed tend to run on the smaller end of the range for gas port size. I have read elsewear that some civilian manufactures tend to run slightly larger gas ports to allow for more reliable funtion with the common cheaper quality (lighter loaded) ammo out there. Sounds like the defense manufacturers such as Colt, LMT, etc. tend to run their gas ports on the smaller end of the range due to the higher quality ammo the weapons are more likely to run (xm193, m193, m855, etc.) which would run the weapon just fine.

Am I on the right track here? Can you guys confirm? Deny? Other info?



Non military barrel suppliers tend to chamber on the tighter cross wall dimensions, requiring more gas to the system to get the spent case out of the chamber. In causes where you have both tight, and rougher chamber conditions, some manufacturers will increase the gas port even more of these increased sizes.

So no, with the slightly wider cross wall dimensions of the military chambers (colt and FN that are good for 3MOA), you don't need all the extra gas going to the gas system.