Grendel Upper: Oversized or standard barrel extension tunnel

[Bonas]

I’ve used 242 to bed a lot of barrels, including ones that had already been installed in uppers and weren’t shooting, then tightened the groups up demonstratively after bedding.

It just acts as a filler/harmonic dampener. The Army Marksmanship Unit did a lot of testing with this back in the day over years and found that bedding the barrels absolutely tightened up their groups.

I’m not looking to form a strong adhesive bond between the upper and extension, just eliminating voids and the subsequent vibration and inconsistent behavior of a loose barrel/upper joint.

I’ve heard others suggest using Green Loc-tite 620, which won’t hurt other than when you try to separate them.

Some high-end builders have used one of the Red Loc-tite compounds as well.

Green 620 is better than Red-271 for the Gas Block and Gas Tube when bedding those due to its heat tolerance.

Joe Carlos with the AMU settled on retaining compound, specifically Loctite 620, not threadlocker. The max service temperature for Loctite 242 is 300 degrees. This is a link to an abstract of some of Carlos's work:

https://www.thefreelibrary.com/The+relat...0321900231

I've read Carlos's papers. Most people credit the idea of bedding barrel extensions with "Loctite" to Carlos's work, from what I've seen. He has migrated from bedding and shimming to using oversized barrel extensions. He discusses "red" versus "green" Loctite in the abstract linked to above. I think the idea that thread locker makes a useful bedding compound might arise from some people thinking of "Loctite" as just thread locker instead of what it is; a brand name for a full line of adhesives made by Henkel.

I recommend anyone who wants to bed an extension with a Loctite brand adhesive to use one of Henkel's products that the manufacturer recommends for high temperatures and is designed to fill gaps. That's 620 or one of the other retaining compounds in the 600 series. Threadlocker isn't designed to fill voids.

But, like I said before, I don't bed at all anymore. I just shim to get a press fit. My last good barrel appeared to come with an oversized extension, which was great.