12" 6.5 Grendel Update 5 month Review

[Disciple]

Gas blocks get bedded per recommendations from Bill Alexander after he fleet-tested his initial rifles. I also bed the gas tubes and ensure proper alignment with the tube and the carrier key. Then there are all the torque specs for muzzle devices ( I want as low of a torque value as possible at the muzzle per AMU lessons-learned), barrel nut torque (different for barrel weight), and scope rings.

Is the gas block betting for accuracy? What is the procedure, and do you also pin it? If you use green retaining compound do you consider this a permanent installation or do you have a method to remove that?

I have read that gas tubes self-seal with carbon after a bit of use. Is that incorrect, or is that bedding for something other than sealing?

How do you select barrel nut torque?

[LRRPF52]

Is the gas block betting for accuracy? What is the procedure, and do you also pin it? If you use green retaining compound do you consider this a permanent installation or do you have a method to remove that?

I have read that gas tubes self-seal with carbon after a bit of use. Is that incorrect, or is that bedding for something other than sealing?

How do you select barrel nut torque?

Gas Blocks
Gas block bedding is for tight seal and consistent resonant harmonics, versus erratic vibrations between the block and barrel. AMU testing and other precision shops learned that long ago.

I dimple and thread-lock the set screws, but pinning would be better. The problem with over 95% of the gas blocks on the market is that they are made for DIY assembly, not tight gas seals.

If they were made correctly with interference fit, then nobody would be able to assemble their DIY builds at home with minimal tools.

Manufacturers who use an interference press-fit have jigs and tooling that enable them to install blocks tightly.

The Knight’s Armament Mod 2 gas system is great in this regard, since it doesn’t require any jigs or special tooling, but does require their thread journal for the front lock nut and a 12 o’clock notched shoulder for the index nipple on the gas block to align. This is the most effective approach for ease of assembly and gas seal, but with the added and peculiar machining for the barrel that isn’t common to any other AR-15s. I noticed that Q uses the same KAC Mod 2 gas block and lock nut approach on the Honey Badger, which appears to be licensed or a direct copy.

What I do is measure the ID of my blocks and OD of the barrel gas block journal, regardless of what the manufacturers claim. We all assume .750” is .750” but sometimes it isn’t.

Same with ID for blocks, which are often .752” or more for a slip fit.

I prefer it to be tighter.

I measure my gas port center distance from the shoulder on the barrel, then measure the gas hole in the block from the block rear face using the protrusion gauge on my calipers.

I then indicate a mark on the journal if there is offset. You can also shim this to fill the gap, but I prefer blocks that abut to the shoulder with proper gas port/hole alignment.

I install the gas tube with high temp thread locker and pin it, ensuring it isn’t canted, then install the whole block with tube and check for correct alignment with the carrier key.

Gas Tube/Carrier Key Fit
I use the inverted upper receiver method to simulate upward cartridge stack spring pressure on the carrier, with the carrier stripped of all the small parts.

I slide the carrier back and forth feeling for any clipping or binding with the gas tube, until I get smooth sliding between the 2 parts, and mark that clock location with the gas block and journal.

Then I install the gas block to the datum reference points using either a heavy brass slip-over cylinder block with a bore in it, or high temp thread-locker on a slip-fit block.

You can also use a drill press for a compression tool to press-fit the block with a fixture or metal block that will allow you to place axial pressure on the block alongside or over the barrel.

Barrel Nut Torque
For barrel nut torque, I go by feel after doing them for over 30 years, but AMU found that lighter torque in the 35-40ft-lbs region worked better on heavy barrels, and 60ft-lbs+ worked better on government profile barrels. Joe Carlos has a great video series on accurizing the AR-15 based on many years of fleet data as an AMU gunsmith. I have a torque wrench, but don’t use it for most of my barrel nut torquing, especially if the nut requires gas tube alignment. For guys that don’t have a lot of upper body strength or much experience with assembly, I recommend using the torque wrench to at least know where your range is. This handguard and barrel nut combo don’t have teeth like a GI nut, so it would be advisable to use a torque wrench.

Muzzle Devices
For muzzle devices, I use Rocksett with minimal torque, especially for suppressed like this one (some suppressors and muzzle devices will add torque to the device with each round fired due to helical tines, so Rocksett is something you can use to address that). If you have incrementally-increasing torque value added with each shot, it will add inconsistent values to the end of your muzzle and affect accuracy. We saw that with a rifle manufacturer I used to do accuracy-testing for, when they were using Smith Enterprises Vortex flash hiders with the helical tines. You could barely hand-tighten that device, fire the rifle 10 times, and it required a cheater bar to get off after that.

I installed the TBAC CB Flash Hider for this 12” and run it suppressed exclusively. The Bootleg adjustable gas carrier, extra power action/recoil spring, and standard 2.9oz carbine buffer are things I see as essential for making that all work together. I measured ejection pattern at the range 2 months ago and it was 4:30 with the setting on fully suppressed (choked). The gun behaves very well as it is set-up currently, with proper feeding, extraction, and ejection.

I apply lessons-learned from:

AMU gunsmiths fleet testing
Bill Alexander/Alexander Arms fleet testing
ARDEC publications on the various test and failure points of the AR-15/M16/M4/M4A1/SOPMOD/PIP
Decades of personal experience working with this rifle design since the 1980s

I’m always trying to learn something new about it though. When I assemble an AR-15 nowadays, it doesn’t even feel like I’m working on the same gun as I did 20 years ago, even if I had all the same parts to start with. My approach is that different.

[Funetes5222]

Great report on this build been looking into 6.5 Grendel last couple months and 6 arc. After reading this I’m full speed ahead on my Grendel build even ordered the faxon 12 inch barrel with the holiday deals going on I got a good deal.

[HKGuns]

I'm going to try to radius my Grendel ejector like you have pictured above. I have 5 different types of Grendel mags and I still have intermittent feeding problems which leaves me not able to trust the rifle.

I even bought a replacement type II bolt from Alexander Arms, which was also supposed to solve my issues. It helped, but didn't completely eradicate the feeding problems.

[caporider]

I'm going to try to radius my Grendel ejector like you have pictured above. I have 5 different types of Grendel mags and I still have intermittent feeding problems which leaves me not able to trust the rifle.

I even bought a replacement type II bolt from Alexander Arms, which was also supposed to solve my issues. It helped, but didn't completely eradicate the feeding problems.

I radiused my extractor and I removed the factory extractor spring o-ring (my extractor had both the ring and the insert - kinda redundant). Doing both of these solved my feeding issues.

[HKGuns]

I radiused my extractor and I removed the factory extractor spring o-ring (my extractor had both the ring and the insert - kinda redundant). Doing both of these solved my feeding issues.

I remove that damned doughnut from all my rifles, so that part is done. Ejector radius is definitely worth a shot.

I think I got it done on my 18V Bosch cordless Lathe.

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