LRRPF52 Wrote:Piston set-up and an untorqued barrel from the extension are where you need to look before even worrying about accuracy.

Any time you're torquing on a muzzle device, the barrel needs to be isolated, not clamping force on the upper, which is a separate part.

I wouldn't have high expectations from an external op-rod gun in accuracy, unless the gas system was pinned and the barrel bedded really well.

Short story is that there are major issues with a lot of the components and assembly methods you used, before we can even talk about the barrel's accuracy potential.

Another thing with Grendel AR15s is that you have to shoot them differently than .223 ARs in most cases, since the projectile weight and recoil make the gun handle a bit differently as to positional shift.

c3k Wrote:I cannot emphasize how easily the barrel came out of the extension.

c3k Wrote:However, he did reply that the barrel should be torqued to 120 lb ft into the extension and to use green loctite

koden Wrote:Barrels can not rotate in the barrel extension after they are assembled. Once they are installed they are pinned to the barrel.



They cant rotate without shearing the steel pin. If your barrel rotated in the extension then it was not assembled correctly by the barrel manufacturer. The pin was probably not pressed in properly or the barrel was not drilled deep enough for the pin. Can you post a pic of the barrel extension and pin? It would be interesting to see how far the pin is protruding from the barrel extension compared to the pic above.



I cant believe Alexander Arms would give you this advice. It needs to be headspaced, timed and pinned.

Kswhitetails Wrote:Love it when the answers are easy. Hard to imagine AA let this one through, but if they're standing behind it, I'm positive they're not the first to deal with one or more instances of this particular issue. A buddy of mine took his BCM apart, and said that the pin would come out of the extension when the barrel was rolled over. It happens, somehow.

Glad to see you've got some resolution.

c3k Wrote:Thanks for looking at my thread. I'm happy to hear of better assembly methods (but, I didn't really mention mine, did I?), and am a neophyte to the AR build world with only a dozen to 15 builds. Certainly nowhere near your level.

When I UNSCREWED the muzzle device, I used the upper block. Why? Well, I certainly did not expect it to spin the barrel off. I used my magpul bevblock when I tightened it on, back in the spring.

I've built and used 5.56 and .223 Wylde ARs with external op-rods. I certainly did not expect the poor accuracy from my first 6.5G that I saw.

When I get the barrel back or if Alexander Arms replaces it, I'll put it together again (using my assembly methods, as good or bad as they may be ), and report back with some range pix.

Without getting too expensive, which of my components would you change, and why?

Thanks,
Ken

c3k Wrote:Yeah...all this.

I sent it in, so no pix. The extension pin only flattened the threads on the barrel. The pin did not protrude any more or less than other barrel extension pins I've seen. It certainly did not interfere with the barrel nut being spun on.

There was NO drilling. The pin looked like it had been tapped into place and it flattened a few threads. It may have been pressed, or it may have been lightly hammered. Regardless, it obviously was not securing the extension to the barrel.

LRRPF52 Wrote:If you use a piston system, the gas block really needs to be securely mounted to the journal, as in pinned with no gas leakage around the seal of the ID of the block and the OD of the barrel.

The barrel needs to be bedded into the upper, either using thermo-fit, compound, or shim stock with compound.

The upper receiver face needs to be square, which is often out of square after anodizing inconsistencies in surface thickness.

With the combined effects of a larger projectile and the asymmetric forces imparted on the rifle from the external piston operation, we normally see measurable and predictable accuracy loss on piston guns.

Some of the more dedicated companies who invested a lot into making their piston guns as accurate as possible had to do a lot of tweaks across the rifle to get them reasonably accurate, the main 2 off the tip of my head being Hk and PWS.

DNS has had good accuracy reports from his piston set-up, not sure what he's using exactly, but he's also shooting suppressed.

I always use the Stoner Internal Expansion gas system, as it is inherently more accurate with minimal asymmetric forces exerted on the barrel.

I check/square my uppers and bed my barrels, bed the gas blocks, balance the torque on the muzzle device, balance the torque on the barrel nut, use equal torque on all my rings, correct torque on the scope base fasteners, I like tight-fitting receivers personally, a great trigger, a bunch of little tweaks with the bolt, ejector, extractor, barrel extension, and certain shooting techniques that I don't need to do with the .223 AR.

Herzo Wrote:Having been in the machining/manufacturing arena for over 60 years, I would be astonished if the process were not:
1. chamber the machined barrel
2. Machine the barrel extension
3. Broach the extension
4. Install the index pin in the extension such that it is clocked (or timed) with the broached feature.
5. Torque the extension to the barrel.
6. Verify the headspace.
7. Drill the gas port in alignment with the index pin.

To attempt to rely on a pair of machined threads to reliably establish a radial alignment is something I would expect from a rookie engineer.

Most likely, AA does not want to invest $100 in a new barrel assembly to make it right. The odds of finding another machined extension assembly that will align correctly and maintain headspace are 2 -- slim and none.

Kswhitetails Wrote:Well said Rabid. Yes, AA, we are. I'm surprised OP, from what I've read (assuming - as we must these days - that everything you've said is 100+) you've been patient enough. It may be time to send them a "nasty" Ashley Judd pic and demand satisfaction.