308 AR platform gas port size?

[AlabamaRoscoe]

What is the proper gas port diameter for a 20" barrel chambered in .308 with a rifle length gas tube?

Having major issues with cycling or lack of cycling. Rifle will sometimes eject the spent round and sometimes it wont but it will never chamber a new round.

I've changed the buffer, tube and buffer spring without any noticeable difference. This weekend i'm going to remove the gas block and see if that might be the culprit.

 

[BBBBill]

I've seen various port diameters published, some with a different port diameter for different barrel diameters. Such as .086 for a .625" diameter 20" barrel and .093" for a .705" diameter 20" barrel. I personally seen one that was .080" diameter.

Bottom line - ensure that you gas block is properly lined up with the gas port radially and longitudinally, and that it seals (no gas block ID/barrel OD mismatch). Also ensure that the gas tube extends the correct distance into the upper and far enough into the bolt carrier gas key. If that is good, then you can start thinking about port diameter issues.

 

[Blue68f100]

Rifle length (12.125" from Barrel torque shoulder) 0.0935"-0.980", same as AR-15's.

You should have a min of 5" of barrel after the port for proper function.

 

[AlabamaRoscoe]

I've seen various port diameters published, some with a different port diameter for different barrel diameters. Such as .086 for a .625" diameter 20" barrel and .093" for a .705" diameter 20" barrel. I personally seen one that was .080" diameter.

Bottom line - ensure that you gas block is properly lined up with the gas port radially and longitudinally, and that it seals (no gas block ID/barrel OD mismatch). Also ensure that the gas tube extends the correct distance into the upper and far enough into the bolt carrier gas key. If that is good, then you can start thinking about port diameter issues.

Will do! This 308 build has been a challenge!

 

[AlabamaRoscoe]

Rifle length (12.125" from Barrel torque shoulder) 0.0935"-0.980", same as AR-15's.

You should have a min of 5" of barrel after the port for proper function.

Thanks. Google is all over the place with port sizes. I'm not home to measure the distance but i believe its well over 5 inches.

 

[AlabamaRoscoe]

Will do! This 308 build has been a challenge!

I've seen various port diameters published, some with a different port diameter for different barrel diameters. Such as .086 for a .625" diameter 20" barrel and .093" for a .705" diameter 20" barrel. I personally seen one that was .080" diameter.

Bottom line - ensure that you gas block is properly lined up with the gas port radially and longitudinally, and that it seals (no gas block ID/barrel OD mismatch). Also ensure that the gas tube extends the correct distance into the upper and far enough into the bolt carrier gas key. If that is good, then you can start thinking about port diameter issues.

How far should the gas tube extend? I watched a video today. Half way between the "half moon" cutout in lower? I know thats not the correct word. Cant recall what its called now.

 

[Blue68f100]

If you got some pin gauges to measure the port size? If not a set of Number drill will get you close.

I've found metal shavings in one years ago.

There is no standard spec like the AR-15. There are 2 different specs used for AR-LR-10's. The 6.5 CM barrel that I purchased years ago had the same problem. There are different strength recoil buffers springs as different weights used. I ended up opening my port up on my LR-10 barrel to get it to function as it should.

The DPMS spec LR-10 use a larger port.

 

[AlabamaRoscoe]

If you got some pin gauges to measure the port size? If not a set of Number drill will get you close.

I've found metal shavings in one years ago.

There is no standard spec like the AR-15. There are 2 different specs used for AR-LR-10's. The 6.5 CM barrel that I purchased years ago had the same problem. There are different strength recoil buffers springs as different weights used. I ended up opening my port up on my LR-10 barrel to get it to function as it should.

The DPMS spec LR-10 use a larger port.

Mine is the LR-308. What I was sold. I believe the DPMS Gen1 is the same or supposed to be the same. Even some of those have differences
Yea, thats the whole problem. Nothing is anywhere close to standard.

 

[AlabamaRoscoe]

If you got some pin gauges to measure the port size? If not a set of Number drill will get you close.

I've found metal shavings in one years ago.

There is no standard spec like the AR-15. There are 2 different specs used for AR-LR-10's. The 6.5 CM barrel that I purchased years ago had the same problem. There are different strength recoil buffers springs as different weights used. I ended up opening my port up on my LR-10 barrel to get it to function as it should.

The DPMS spec LR-10 use a larger port.

I purchased a Odin works adjustable buffer. Have yet to try that out. Really cant figure out where to start with that.

Bought a Aero buffer, tube and spring that solved nothing. May be slightly worse with that spring and buffer combo I know its been aggravating.

 

[Blue68f100]

Are you running a AR-15 buffer spring or AR-10? They look identical, except one is stronger. Try a AR15 buffer spring, with a H1 buffer and see where that leads you.

Is your BCG a std/heavy one or light weight version?

Is the bolt piston rings adjusted so the gap in not aligned up. Or are you using JPI 1 piece ring.

Are you shooting factory ammo?

 

[BBBBill]

How far should the gas tube extend? I watched a video today. Half way between the "half moon" cutout in lower?

One word answer, yes.

As you've noted, there are no common standards for the AR-10/LR-308/whatever else it may be called. The original Stoner designed Armalite AR-10 had a 15-1/2" gas tube on the 20" barrel (only barrel length made at the time). I think I recall that DPMS used a 15-1/4" length on their guns. Either way, the gas tube should end even with the middle of the cam pin relief cut in the upper. That will create the correct "dwell time" (not the correct term) for the tube inside the gas key of the bolt carrier. In other words it will be inside the gas key long enough to deliver the amount of gas needed to operate the action assuming that everything else is correct.

 

[AlabamaRoscoe]

Are you running a AR-15 buffer spring or AR-10? They look identical, except one is stronger. Try a AR15 buffer spring, with a H1 buffer and see where that leads you.

Is your BCG a std/heavy one or light weight version?

Is the bolt piston rings adjusted so the gap in not aligned up. Or are you using JPI 1 piece ring.

Are you shooting factory ammo?

I'm running AR10 buffer and spring from AERO. The BCG is a standard weight.

Not sure what a bolt piston ring is or a JP1 ring. I will have to research that further.

Yes ammo is factory. Tried three different brands. Hornady Black 168 gr., Federal Premium 168 gr and some cheaper 150 gr.

 

[Blue68f100]

I'm referring to the piston rings on the bolt. If the gap on all three rings are in alignment you will pass a bunch of gas.

 

[AlabamaRoscoe]

Gas tube looks way short! It measures 15 1/4 Thoughts?

 

[Blue68f100]

Your tube does look short. I had to dig mine out of the safe to take a look. Mine extends about 1" from the mag opening. The expanded tip is setting in the middle of the cutout/notch.

It looks like time to do some measuring to make sure the gas port is in the correct spot. Then confirm the gas tube length.

 

[AlabamaRoscoe]

Your tube does look short. I had to dig mine out of the safe to take a look. Mine extends about 1" from the mag opening. The expanded tip is setting in the middle of the cutout/notch.

It looks like time to do some measuring to make sure the gas port is in the correct spot. Then confirm the gas tube length.

Gas tube is 15 1/4. There is no way to move gas port.

 

[AlabamaRoscoe]

Also there was no pin going through port to hold tube.

 

[Blue68f100]

Measure the length from gas port to the front or the barrel extension nut. This will tell you if the gas port was drilled in the correct position. Ref my Post #3.

Without the pin you may or may not have a gas leak. Some for the end plugs are solid, some are not. If you see carbon at the pin hole than it was leaking. The barrel port and gas port looks to be centered well.

 

[BBBBill]

It appears that you have a DPMS spec gas tube on an AR-10 type barrel. That's what I meant by different standards from different manufacturers above. Your tube needs to be longer to get it in the middle of the bolt cam pin clearance cut. Whose parts are you using? Barrel? Gas tube? And do pin the gas tube/gas block.

 

[AlabamaRoscoe]

It appears that you have a DPMS spec gas tube on an AR-10 type barrel. That's what I meant by different standards from different manufacturers above. Your tube needs to be longer to get it in the middle of the bolt cam pin clearance cut. Whose parts are you using? Barrel? Gas tube? And do pin the gas tube/gas block.

I purchased a complete upper from Delta Team Tactical. Looks like I need a 15 1/2 inch gas tube. to get centered. Not sure where to find one of those. Most all I see are 15 1/4"

 

[AlabamaRoscoe]

Measure the length from gas port to the front or the barrel extension nut. This will tell you if the gas port was drilled in the correct position. Ref my Post #3.

Without the pin you may or may not have a gas leak. Some for the end plugs are solid, some are not. If you see carbon at the pin hole than it was leaking. The barrel port and gas port looks to be centered well.

There is no doubt I had a gas leak without the pin installed the gas tube was sliding back and forth. I waiting on a pin to arrive. Will test it when it comes in.

 

[BBBBill]

I purchased a complete upper from Delta Team Tactical. Looks like I need a 15 1/2 inch gas tube. to get centered. Not sure where to find one of those. Most all I see are 15 1/4"

Here ya go -
Primary Arms - ArmaLite Rifle Length Gas Tube (primaryarms.com)
Wilson Gas Tube | Rifle Length | AR10 (shopwilsoncombat.com)

Quote from the website - "This 15.5” long, extended stainless steel gas tube is what we use in our large bore AR10 builds. The slightly longer length gives you a better seal with AR10/LR308/SR25 style gas keys for improved reliability and performance."

 

[AlabamaRoscoe]

Here ya go -
Primary Arms - ArmaLite Rifle Length Gas Tube (primaryarms.com)
Wilson Gas Tube | Rifle Length | AR10 (shopwilsoncombat.com)

Quote from the website - "This 15.5” long, extended stainless steel gas tube is what we use in our large bore AR10 builds. The slightly longer length gives you a better seal with AR10/LR308/SR25 style gas keys for improved reliability and performance."

Thanks!! Ordered the Wilson Combat.

 

[David Hoback]

Sounds like you should be GTG now. This is a big reason I’m not so fond of the 10’s. There are two different platforms: the DPMS LR308 & Armalite AR10, and even within each platform, there is little standardization. So much of the time companies make parts according their OWN set of specifications. And are typically very “Hush-Hush” on the specifications of said parts. The builder is left with the choice of either buying everything from one manufacturer, or trying to match up parts that work together.

Very different from the AR15 platform that just works! I’ve had so little problem over almost 2 decades & countless builds(not really countless, but seems like it, LOL!). You build an AR15 using each part from a separate manufacturer & as long as they are all high quality, US made parts chances are, it’s going to perform fantastically.

 

[AlabamaRoscoe]

Problem solved! The Wilson combat 15 1/2" gas tube did the trick. Runs great now! Thanks for all the assistance