Gas Port Sizes/Gas Block Question

[Brandi]

I have a Loki Weapons System .264 LBC with an 18" stainless barrel with a midlength gas system. Someone told me that it's probably a Liberty barrel because that's what Loki used on their build but since the company is long gone there is no one to ask. When I first got the gun it had all kinds of issues with short stroking and such. It had FTE's like crazy and dented the crap out of the brass. I smoothed out some spots on the magazine and lightly touched the bolt face which seemed to fix everything. Fast forward to last month when I'm shooting and notice there's a significant gas leak at the block. Because of the free float tube I never saw any evidence of it until it started showing out the end of the tube. Since I already had a new handguard purchased (no way to attach things on the Hogue Free Float) I decided it was the perfect time to swap handguards and fix the leak at the same time.

Of course the Hogue handguard had been attached with some kind of alien technology adhesive and had to be sawed off (that's a whole different story). Anywho, the lo-pro gas block was also on insanely tight and had to be removed with heat and a press. After finally gaining access to the gas port I measured it and it ranged from .0865 to .0885 with digital calipers depending on where I measured it. It was pretty black and I'm guessing the difference in measurements is erosion. There was a lot of carbon around the hole so I don't know of those measurements might be a bit smaller than actual size or the exact size.

I've been searching the internet all day for information on gas port sizes and have gotten nothing but conflicting information. My question here is, is this gas port the right size? I'm guessing the port is a little big for the lo-pro gas block which is causing the large leak. I'm thinking that an adjustable gas block would be a better choice in this case, what do you recommend? The new MI Gen 3 FF handguard would allow access to the adjustable gas block which is a plus.

Any other recommendations are welcome Much thanks!

[LRRPF52]

Brandi,

An 18" MLGS Grendel should be .076" port diameter. .0865-.0885" is slightly bigger, and will allow more gas volume to flow, which usually creates excessive bolt carrier speed and too early unlocking.

You can do adjustable gas at the block, or at the gas key or carrier if you go with adjustable gas.

Liberty barrels have normally been ported at .0935" for MLGS, which is way too much and needs to be tamed.

FTExtract is the main symptom you will see with an excessively large gas port at Mid-Length, because the case is still obdurated to the chamber wall, while the extractor is trying to rip it out. Since there is way more surface area on the case purchased to the wall and under pressure, versus the small area of the extractor contacting the case rim, the chamber wins that tug-o-war.

With adjustable gas, you will often have a hard time fitting it under slim MI Gen 3 handguards, so look for gas blocks that will clear the ID of that handguard. You could also just replace the carrier key with a Sun Devil adjustable key, which is a lot easier to access and service, or get an adjustable carrier from 2A, but they're pricey ($419).

The Sun Devil ADIGS is $60. http://shop.sundevilsales.com/ADIGS-...-ADIGS-KIT.htm

[Kilco]

I'm not sure what the difference is with inside diameter, but the Odin Works Tunable gas block clears my Midwest Industries Gen 2 SS. Not a ton of room to spare, but it does clear. I don't have any experience with the Gen 3 however.

[StoneHendge]

Brandi,

An 18" MLGS Grendel should be .076" port diameter. .0865-.0885" is slightly bigger, and will allow more gas volume to flow, which usually creates excessive bolt carrier speed and too early unlocking.

You can do adjustable gas at the block, or at the gas key or carrier if you go with adjustable gas.

Liberty barrels have normally been ported at .0935" for MLGS, which is way too much and needs to be tamed.

FTExtract is the main symptom you will see with an excessively large gas port at Mid-Length, because the case is still obdurated to the chamber wall, while the extractor is trying to rip it out. Since there is way more surface area on the case purchased to the wall and under pressure, versus the small area of the extractor contacting the case rim, the chamber wins that tog-o-war.

With adjustable gas, you will often have a hard time fitting it under slim MI Gen 3 handguards, so look for gas blocks that will clear the ID of that handguard. You could also just replace the carrier key with a Sun Devil adjustable key, which is a lot easier to access and service, or get an adjustable carrier from 2A, but they're pricey ($419).

The Sun Devil ADIGS is $60. http://shop.sundevilsales.com/ADIGS-...-ADIGS-KIT.htm

Not for a Grendel, but an adjustable gas key should enable slowing down the bolt unlocking without affecting the amount of rearward force applied to the BCG for cycling, right? I'm getting there, but still dealing with some case head abuse on my 22 Nosler.....

[Brandi]

Thanks all, I'm going to start the search for something adjustable. Much appreciation!

[LRRPF52]

Not for a Grendel, but an adjustable gas key should enable slowing down the bolt unlocking without affecting the amount of rearward force applied to the BCG for cycling, right? I'm getting there, but still dealing with some case head abuse on my 22 Nosler.....

There isn't rearward direct impingement force on the AR15 BCG, but internal expansion in the chamber formed by the bolt tail and gas ring flange, as well as the rear chamber wall in the bolt bore of the carrier.



As the gas expands, it causes the carrier to move rearward since that is the path of least resistance.

An adjustable gas key will limit the volume of flow per time into that Stoner expansion chamber, causing later movement than normal. Basically trying to time the system with choked gas flow back at the key.

Go to the wildcats section and start a threat if you need trouble-shooting with the Nosler.

[Drillboss]

There isn't rearward direct impingement force on the AR15 BCG, but internal expansion in the chamber formed by the bolt tail and gas ring flange, as well as the rear chamber wall in the bolt bore of the carrier.



As the gas expands, it causes the carrier to move rearward since that is the path of least resistance.

I'm not following the explanation. It looks to me like the inside diameter of the gas key tube is the incremental area that provides a net rearward force on the bolt carrier.

[Sticks]

The gas is also exerting force on the bolt pushing it forward...where it is locked in = pushing the carrier back, then the cam pin starts rotating the bolt for unlocking and extraction.

[Drillboss]

The gas is also exerting force on the bolt pushing it forward...where it is locked in = pushing the carrier back, then the cam pin starts rotating the bolt for unlocking and extraction.

I get it now. The initial gas pressure impulse starts the bolt carrier moving back, unlocking the bolt as you said, after which the bolt carrier continues travelling back simply due to inertia.

[LRRPF52]

I'm not following the explanation. It looks to me like the inside diameter of the gas key tube is the incremental area that provides a net rearward force on the bolt carrier.

See the chamber formed by the gas ring flange and the carrier. When the gas rapidly fills that area, it starts pushing outwards against the rear of the bolt and the carrier wall that the bolt tail protrudes through.

That's what starts inertia in the Stoner gas system, whereas on a Ljungman, there is a gas pocket above the carrier where the gas pushes against, causing the carrier to move rearward.

The carrier key on the AR15 is just a path for the gas to travel through until it hits the expansion chamber. Once sufficient inertia has been generated, the carrier moves such that the vent holes will pass over the gas rings, and allow any excess gas pressure to vent out of the ejection port. This is another reason why the AR15 is so debris tolerant, as this gas will blow mud and dirt away from the action upon firing.

An adjustable gas key chokes the amount of gas that can enter the expansion chamber within the timing of gas expansion from the system.

Most adjustable gas blocks choke the gas port under the gas block right above the port on the barrel.

[StoneHendge]

See the chamber formed by the gas ring flange and the carrier. When the gas rapidly fills that area, it starts pushing outwards against the rear of the bolt and the carrier wall that the bolt tail protrudes through.

That's what starts inertia in the Stoner gas system, whereas on a Ljungman, there is a gas pocket above the carrier where the gas pushes against, causing the carrier to move rearward.

The carrier key on the AR15 is just a path for the gas to travel through until it hits the expansion chamber. Once sufficient inertia has been generated, the carrier moves such that the vent holes will pass over the gas rings, and allow any excess gas pressure to vent out of the ejection port. This is another reason why the AR15 is so debris tolerant, as this gas will blow mud and dirt away from the action upon firing.

An adjustable gas key chokes the amount of gas that can enter the expansion chamber within the timing of gas expansion from the system.

Most adjustable gas blocks choke the gas port under the gas block right above the port on the barrel.

I think I'm getting there - sometimes you remind me of my mechanic talking about the diesel engines in my busses - I follow and then ...... whoosh! Over my head .

If I have an adjustable gas block and am at the point where I am just reliably cycling, dialing down an adjustable gas key should enable me slow my bolt down and delay rotation and hence, hopefully stop the extractor from ripping at the rim before the case relaxes. But would that have the effect of dialing down the gas block and impact cycling? Or would it delay the cycling and still provide sufficient force to cycle?

[Drillboss]

See the chamber formed by the gas ring flange and the carrier. When the gas rapidly fills that area, it starts pushing outwards against the rear of the bolt and the carrier wall that the bolt tail protrudes through.

That's what starts inertia in the Stoner gas system, whereas on a Ljungman, there is a gas pocket above the carrier where the gas pushes against, causing the carrier to move rearward.

The carrier key on the AR15 is just a path for the gas to travel through until it hits the expansion chamber. Once sufficient inertia has been generated, the carrier moves such that the vent holes will pass over the gas rings, and allow any excess gas pressure to vent out of the ejection port. This is another reason why the AR15 is so debris tolerant, as this gas will blow mud and dirt away from the action upon firing.

An adjustable gas key chokes the amount of gas that can enter the expansion chamber within the timing of gas expansion from the system.

Most adjustable gas blocks choke the gas port under the gas block right above the port on the barrel.

After a little research, I see it all now. When I first looked at the drawing, I pictured it with the bolt extended forward with the area that the pressure is acting on the same both rearward and forward (except for the gas key diameter). Now I understand that when the bolt is all the way forward, the vent holes in the side of the carrier are opened and the gas key has probably moved off of the gas tube, so pressure isn't acting any longer and it's all inertia from there.

Thanks for the insight.