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School me on Gas port locations


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Okay, I know there is a slight difference between Armalite and dpms/ SASS gas tube lengths.  I am in the process of pairing up a barrel length and ff handguard and would like to atleast partially cover the gas block, I'm set on  a 13" Troy extreme battlerail and I plan on running a low profile adj gas block.  Ive been homing in on a FA 16" ultra light barrel with a rifle length gas system. Now I understand that not every manufacturer is going to put their gas port in the "exact" same spot as everyone else but could you tell me the aprox location of the gas port in the rilfe, mid, carbine and pistol length system? Also where are they typically measured from the breech or receiver face?  Been searching around and didnt find any good breakdowns. Maybe someone could post up a good lesson for us noo-b's and pin it like the buffer thread? 


Forgive me as I am at work with nothing better to do. <dontknow>

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SYSTEM       Barrel length       Port distance


PISTOL          >  10"                      4 "


Carbine        10 - 18 "                    7 "


MID                  > - 20 "                   9"


Rifle              16 >                         12 "


I would think the measurements would be from the chamber , but could be wrong & not all barrel & rifle manufacturers go by this , but most do.


Your 13 " hand guard should cover the low profile gas block . I personally like them exposed for several reasons , one being able to service or replace the gas block, with out removing the hand guard.

Edited by survivalshop
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For sure!  Pinky is a 16" midlength, and that thing might look funny, but it's one hell of a shooter.  The other 16" midlength I have is a precision rig, the Spike's billet set with the Wilson/Nordic barrel.  That thing is smoooooooooth.


Carbine gas system was a direct development of rifle gas system on the A1/A2 with the 20" barrel.  Dwell time - distance from the end of the barrel, back to the gas port location.  The amount of time (or distance) that gas is entering the gas port, before the bullet leaves the barrel.  It's almost identical on the 20" A1/A2 barrel (from end of barrel back to gas port location) as it is on the 14.5" barrel carbine.  End of carbine barrel back to the gas port location, of the carbine gas system.


Civilian-legal barrels must be a minimum length of 16", so carbine gas systems in a 16" barrel have longer dwell time.  A 16" AR with carbine gas feels a little "more harsh" or has "more recoil" than the military 14.5" carbine - because of that extra 1.5" of barrel.  More dwell time.


Midlength gas systems started out as an alternative on a 16" civilian-legal barrel, to replicate that magic amount of dwell time of the A1/A2, and the military carbine with the 14.5" barrel.

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