Barrel Gas Port Locations

[stevegun 1]

A few questions for the vast knowledge base of the horde. Can anyone provide info/drawings on which datum is used on the different length gas systems for the AR platform? Also, what is the relationship between the gas port location and the barrel OD (.750", .875", etc.) and if there's a standard for the length of the OD that the gas block mounts to? Thank you in advance for all help provided.

[LRRPF52]

From the face of the barrel extension flange-

MLGS:
To the Shoulder: 8.75"
To the gas port: 9.00"

RLGS:
To the shoulder: 12.125"
To the gas port: 12.375"

Location is not dependent on gas block journal OD. Gas port diameter can be changed for gas block journal OD. The goal is to cut the gas port to the minimum size for the gas system length and average range of projectile/charge/temperature combinations for reliability, without over-gassing the operating system.

[stevegun 1]

LRRPF52, thank you. If I read this correctly, the rear shoulder of the gas block journal starts .250" before the gas port location as the dimensions progresses to the muzzle? Typicaly, how long is the average gas block journal? Do these dimensions "start" at the face of the upper, barrel extension shoulder or what? Is there any suggested port size for an adjustable gas block? What about the dimensions for a CLGS?

[LRRPF52]

Yes, from the shoulder of the barrel where it steps down to the gas block journal, to the gas port center is typically .250". Gas block journal lengths differ depending on what type of gas block will be used, and the two main divergences here are:

* Standard FSB (military-type or similar using taper or straight pins)
* Low profile or non integral front sight gas block

Either way, most barrels will have a 1.900" journal length, from the shoulder to where the barrel profile steps down. A .750" gas block journal usually steps to a .725" barrel fore end, until getting near the muzzle.

The gas system lengths I posted earlier are measurements from the face of the barrel extension flange (some call it a torque ring).

[maverick5582]

[IMG][/IMG]

This is an image should help some. My dimensions may be slightly different for my rifle length gas system.

[stevegun 1]

Thanks Maverick, nice illustration.

LRRPF52, in almost all the barrel/gas block combinations I've installed the gas block does not butt up against the barrel shoulder, short from .010-.020". When profiling a barrel is there any advantage to designing the gas block journal so that a given gas block will "butt" up against the shoulder? When designing a gas block, shoulder the rear be designed to be short of that .250" dimension so it would fit all gas block journals?

[LRRPF52]

The reason why gas blocks and FSB's don't abut with the shoulder is because the gas port location was made with the intent of the handguards being captivated by a handguard cap. Since we don't really use handguard caps on precision rifles, they being free-floated, there is no usefulness to this dimension, unless someone wants to take a higher end barrel, and turn it into a service rilfe look-alike.

I personally lean towards the idea that mating the gas block up with the shoulder would probably be better for harmonics, and this is how I had GA Precision do my .260 Rem AR10. They profiled the shoulder to match the OD of the gas block OD as well, and it has a nice, blended fit, feel, and finish.

If I was profiling 18" and longer Grendel barrels that are clearly being used in free-float AR15's, I would get rid of the silly handguard cap space. Even if someone wanted to take their Jaguar and turn it into a Chevy, they could just machine down the rear flat of the military FSB and still make it fit just fine.

Some of the gas blocks will allow you to abut them to the shoulder, because their gas port hole is large enough to still cover the barrel gas port, even of the holes are not concentric to each other. If you do set up a gun like that, you then need to trouble-shoot the gas tube where it inserts into the carrier key, ensuring that everything still allows for the carrier to go into battery without binding on the gas tube flange.

[NugginFutz]

The simplest, most direct solution for me has always been a .025" feeler gauge. I don't argue with the gas block, and it never beats me up at the range.