question on bedding the barrel extension into the upper

[A5BLASTER]

I’m not a mechanical engineer just an industrial mechanic.

The expansion coefficient of aluminum is 2.4 times the coefficient of steel. A barrel with a loose fit of .002” will grow to .0048” , with the dissimilar metals. Couple that with when a thread fastener like a bolt or in this case a threaded receiver grows from thermal expansion if it can’t grow radially out due to a restriction ( the barrel nut) then it will grow axially and loosen thread engagement with your barrel nut. This loosing of engagement will reduce your clamping force allowing a loose fitted barrel to be able to wander. This may not matter in a basic service rifle with 4” groups but if you are tryin to squeeze every last bit of accuracy out of a rifle then it can matter a great deal.

This is where 600 series loctite comes in to play, not 200 series which are mostly thread lockers. 600 series or retaining compounds that are designed to fill in fits of so many thousandths of an inch.

None of what I said is controversial or debated. It’s a common understanding in the firearms community. That’s why companies like BCM and others make tighter tolerances for the upper receivers. It’s why it’s a common practice to bed rifle actions.

Loctite on an AR barrel only makes sense with a loose fit. I personally have seen the difference it makes in a loose fitting upper.

I stopped using the glue method a good time back.

Step up to steel shim stock and do a heat and freeze compression fit up and have the best fit up possible.