Grendel Upper: Oversized or standard barrel extension tunnel

[LRRPF52]

Loctite 620 and 242 are wildly different adhesives. 620 is a high-temperature retaining compound for cylindrical slip fits. 242 is thread locker. Thread locker isn't going to do much of anything between a barrel extension and receiver. Thread locker doesn't have any structural properties that could serve a "bedding" function. 620 does, though. 620 isn't coming loose unless it's heated to 500 degrees, when used properly. It's very strong.

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.