question on bedding the barrel extension into the upper

[PVBoom]

If you want to ensure a tight fit, you may want to consider using a BCM upper receiver. They intentionally size the receiver on the small side of the tolerances so that you have to heat it up to fit the barrel extension. I bought their blemished receivers at $60 and used them for my 11.5" 5.56 pistol and 18" 6.5 Grendel builds. For the first one, I held the threads over the stove, and on the second I used a heat-gun - both methods worked just fine.

However, you may have trouble truing the face, as the PTG facing tool that I have wouldn't fit in either of the BCM uppers I have. If you want a true face that's a thermal fit without buying extra tools, you might want to look at JP Enterprises or Precision Firearms billet receivers. They're pricey, but they have a good reputation.

This.

I was waiting for a while, unsuccessfully for BCM uppers to come back in stock. Then I found someone on arf.com who was unloading a 20" Lilja Grendel w/ GB, tube, bolt and he had an extra (used) BCM upper. I bought them. Before it shipped, he offered to lap the face of the receiver. I said I had the Brownells tool, he said he did too, but it doesn't fit, so I had him do it.

I froze the barrel overnight and used a heatgun on the upper for about 10 seconds, it slid in smooth on the first attempt.

[Bobjones]

A little bit of a pricey tool but works great at spot heating for breaking a locktite bond. I'm a bit of a tool junkie.......

https://www.theinductor.com/induction-he...d-700-110v

[Klaus Von Richter]

please explain to me; if the barrel is tightened to the upper with barrel nut @ 40ft-lbs, what difference does your loctite make after that?......

bedding is useless operation; feel free to prove wrong, but using mechanical engineering arguments and not your observations which can have many variables to it.

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 are 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.


Edited for spelling mistake

[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.

[tdbru]

A5B,
question for you. as far as end accuracy goes, do you see the same improvement whether using 600 series loctite or a steel shim interference fit? will either method deliver the goods?
thanks,
-tdbru

[Klaus Von Richter]

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.

Steel shim would be just as good or better than loctite. Especially if you are getting a interference fit with shim stock.

[A5BLASTER]

A5B,
question for you. as far as end accuracy goes, do you see the same improvement whether using 600 series loctite or a steel shim interference fit? will either method deliver the goods?
thanks,
-tdbru

Honestly I can't remember what my groups were like in comparison to how they are now that I started using the shim stock and heat and freeze method.

I mean I know I had sub moa groups but honestly can't say if they were as consistent as they are when using shim stock.

With the method I'm using now and doing a well fitted and balanced build torque wise, my rifles seem to hold much better and more consistent groups then most I see. But my barrel choices have a lot to do with that as well. Cheapest barrel I run is the sixfivearms barrels.

What I have noticed is when working up handloads my rifles just seem to be less ' picky' then what I see others talking about.

Dont get me wrong the glue method works, but I was never really satisfied with it due to what I do for a living. I knew that a shim and doing a compression fit would be better.

Lapping the receiver face, at first thought help accuracy but now I'm of the mindset. Lapping is for a square bolt lock up and bolt life.

[Growler]

A5B:
What thickness stock did you use?
Any issues keeping the shim material in place while working the cold extension into the heated receiver?

[A5BLASTER]

A5B:
What thickness stock did you use?
Any issues keeping the shim material in place while working the cold extension into the heated receiver?

I have two rolls of it one says 5 thousands and one says its 10 thousands. Just depends on the extension what thickness I use. Mostly use the thinner one.

I haven't had a issue with it moving while installing the cold barrel mainly because I use a couple of small drops of gel super glue to hold the seams down and keep it tight. I do this before putting it in the freezer. You dont want the ends to over lap each other and you dont want the seam on the top/bottom always put it on the side. I also cut a small section out of it to fit around the indexing pin.

[Mustangfreek]

Square the face, shim with stainless shim stock if needed , loctite 620 or such has been my go to way on every upper for awhile now.

A small difference in some and bigger then others.

But I’m ocd and now look at Everything in a build.

[mtnlvr]

What about copper shim?

[Old Bob]

What about copper shim?

I'd say no. You'll get galvanic corrosion between the copper & the steel.

[Les]

If you want to ensure a tight fit, you may want to consider using a BCM upper receiver. They intentionally size the receiver on the small side of the tolerances so that you have to heat it up to fit the barrel extension. I bought their blemished receivers at $60 and used them for my 11.5" 5.56 pistol and 18" 6.5 Grendel builds. For the first one, I held the threads over the stove, and on the second I used a heat-gun - both methods worked just fine.

However, you may have trouble truing the face, as the PTG facing tool that I have wouldn't fit in either of the BCM uppers I have. If you want a true face that's a thermal fit without buying extra tools, you might want to look at JP Enterprises or Precision Firearms billet receivers. They're pricey, but they have a good reputation.

This ^^^^
If you use BCM uppers no bedding or locktite needed or wanted. They are tight. Thanks @DrillBoss for the tip.

[tdbru]

well, i don't have a BCM upper. so i am making the best of what i have to work with. i appreciate all the inputs. good education for me.
thanks,
-tdbru