Anyone using any of the new BCM.....?

[Stickman]

When rings expand with heat, the inside diameter increases.

I think Mark is talking about the extension, as in the barrel extension. When we are talking about a desired tight fit (which I think we all want), heating the barrel extension would be pointless. If we are calling the upper receiver threads an "extension", it would make sense. However, I'm not versed in the threaded section ever being referred to as "the extension".

I think we are all on the same sheet of music with this, just using varying terminology. If someone is heating the barrel extension, to aid in the barrel to receiver fit, I would caution them to slow down and rethink their concepts.

[1168]

You’re right…I misunderstood the question. I mean heating the threads on the upper receiver, not the barrel extension.

[bugsy714]

You’re right…I misunderstood the question. I mean heating the threads on the upper receiver, not the barrel extension.

I think it’s called the receiver extension but yeah the threaded part on the receiver it has heat that little section up with a micro torch or a heat gun it doesn’t take much heat to loosen it up since it’s aluminum then seat that barrel because the cold barrel will cause it to contract almost immediately upon contact


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[georgeib]

FWIW, I've installed a few interference fit barrels to uppers. I've always put the barrel in the freezer for a couple of hours, and put the receiver in the oven. In the hopes that the entire thing would expand evenly and give me a few extra seconds of working time.

It has always worked like a charm. And the times I've needed to subsequently separate the barrel from the upper, it has required pounding it out using a wooden dowel and mallet.

[Disciple]

I think it’s called the receiver extension but yeah the threaded part on the receiver it has heat that little section up with a micro torch or a heat gun it doesn’t take much heat to loosen it up since it’s aluminum then seat that barrel because the cold barrel will cause it to contract almost immediately upon contact

Do you not risk heat degrading the aluminum alloy when using a torch rather than boiling water? I am looking for information on 7075 but for 6061-T6:

An example for the wrought AA6061 alloy in T6/T651 condition is shown in Figure 2a,b. At temperatures above 150 °C, the alloy suffers a loss in strength with deterioration increasing over time. Above 200 °C, the weakening is substantial, and is accompanied by some gain in ductility. Most of the strength reduction induced by exposure to elevated temperatures is permanent, so the loss in strength is not recovered when the material is returned to a lower temperature.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7435424/

[bugsy714]

Do you not risk heat degrading the aluminum alloy when using a torch rather than boiling water? I am looking for information on 7075 but for 6061-T6:



https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7435424/

Yeah you don’t really want to overdo it this isn’t a muscle job but more of a finesse job

The professional builders in my area is use a heat gun


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[1168]

FWIW, I've installed a few interference fit barrels to uppers. I've always put the barrel in the freezer for a couple of hours, and put the receiver in the oven. In the hopes that the entire thing would expand evenly and give me a few extra seconds of working time.

It has always worked like a charm. And the times I've needed to subsequently separate the barrel from the upper, it has required pounding it out using a wooden dowel and mallet.

This is what I do.

[bugsy714]

https://youtu.be/-cOlNGxUhsk


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[markm]

I think we are all on the same sheet of music with this, just using varying terminology. If someone is heating the barrel extension, to aid in the barrel to receiver fit, I would caution them to slow down and rethink their concepts.

This... yeah. I see extension and assume barrel. Yes. Heating the upper would logically help.

[titsonritz]

Sounds like a fun time at Bubba’s basement to me

I used a micro torch to heat the extension and then while it was still hot give it a few good wax with a rubber mallet on the end of the receiver to seed it

I think it’s called the receiver extension but yeah the threaded part on the receiver it has heat that little section up with a micro torch or a heat gun it doesn’t take much heat to loosen it up since it’s aluminum then seat that barrel because the cold barrel will cause it to contract almost immediately upon contact

FYI "Receiver Extension" is the proper term for "buffer tube".