I'm just learning about this thermal fit, where you have to heat the receiver to fit the barrel.

They say the accuracy becomes much better, some saying as good as a bolt action rifle.

It seems only BCM makes these uppers now in 5.56?

If they are so much better, why doesn't every company make their uppers require a thermal fit? Isn't it as easy as making the hole a little smaller?

Should I only buy BCM uppers because they have this option and are that much better? What other company makes upper receivers in 5.56 like this?

This isn't the case, every time.  I have assembled many, many uppers, and have had barrels that slip into a BCM upper without warming it up, or putting the barrel in a freezer.

I have had BCM uppers that absolutely do require it, with every barrel I tried, and then found that my VLTOR uppers required it with that barrel as well.

It is not a 100% rule with the BCM uppers.  The barrels play a part as well.

Tolerances. Not all barrel extensions are the same outter diameter for example.

Sons of liberty gunworks also purposely undersizes their upper receivers.

And a thermal fit is not important, or about the last thing on the list of importance for accuracy, a square face is far more important.

Because it takes time and money to do, and not everyone is interested in paying for it

Hellbender-s-a-k-a-Lostinthewoods-Guide-to-the-Ultimate-AR

Great read, Highly Recommend

iirc hodge does it as well

The beauty of the ar15 is modularity and easy parts replacement. Making parts so tight defeats the intent of the design.

It's also marketing bs that a "thermal fit" barrel is what makes the gun accurate. A square faced upper, in-spec barrel nut properly torqued, quality barrel, and proper bolt engagement combined with good ammo is what gets accuracy.

The barrel and upper are going to heat up during shooting and then what? Your barrel extension surface engagement with the upper are going to fluctuate.

Not to mention the skill and technique of all but benchrest competitors wouldn't ever show a benefit from such a minute detail even if true.

Does the thermal fit cause potential stress on the aluminum upper?

Tolerance stacking, as was mentioned. They manufacturer has no idea what bbl you plan on installing so most build in a small amount of play to ensure fit. We're talking about a few thousandths here.

FWIW my BCM upper didn't require any heat to get a BA barrel into it. I'll be swapping the barrel out in the next few months and would imagine the next one will slip in easily. The upper has seen well over 10k rounds and id think it loosens up ever so slightly over that kind of round count.

That's now how the M16 was originally designed. Modularity is key.

See below



True, and all upper receiver "snout" inner diameters arent the same either. The only way to gaurentee desired tolerances every time is to personally machine both the receiver "snout" bore and barrel extension, holding to desired tolerances or match a custom barrel extension to a pre existing upper using precision measuring and machining. When using one manufacturers upper receiver and another manufacturers barrel (with installed extension) it is impossible to gaurentee desired fit..regardless of manufacturer or money spent. There is a tolerance range and the receiver "snout" id and barrel extension od can be on opposite ends of the specified range, causing a sloppy fit yet still being whithin spec.



Along with JP and others, but that doesnt gaurentee an optimal fit, the barrel extension is the other half of the equation. Even if both components are from the same manufacturers, tolerances would have to be strictly controlled (aerospace grade precision machining and measuring instruments) to thousandths and even half thousandths to gaurentee that fitment everytime and that costs $$$. BAT Machine offers barrel extensions in .0005 (half thousandths) increments to achieve desired fitment between upper receiver "snout" id and barrel extension od on competition uppers.



Id say equally important; and so would Mr. Joe Carlos former U.S Army Reserve Markmanship Unit team armorer/1000 yd service rifle record holder (granted M14/M1A)/one of if not the top service rifle builders in the country. He has found through machine rest testing of hundreds of USAR competition uppers and hundreds of uppers built for competitors, that the barrel nut even if torqued to the upper end of spec is only sufficent to secure the barrel forward to aft, not sufficent to secure the barrel 360° within the receiver "snout" bore. Mr. Carlos has recorded groups showing a 35% reduction in group size (with recorded groups before and after using machine rest) using a combination of green loctite and stainless shim stock to tighten fit. Email him and ask him for the data..
"In closing, there are videos being published one after the other on
these AR building topics.  A lot of those videos contain a fair amount
of misinformation.  If the person in front of the camera can't tell you
how much accuracy gain you can expect and if he can't tell you how large
his sample size was for drawing his conclusions he probably hasn't
performed his due diligence in conducting scientific testing on the
techniques he's promoting. Anyone wishing to discuss these matters
further is encouraged to email me directly: NCC1701@ penn.com"

Is any of this (including receiver face squaring) required for a rifle to function to minimum (milspec) standards or for mag dumps at tin cans?..Of course not, the Stoner rifle was never designed or intended to be a precision rifle, But if your goal is absolute maximum performance in regards to accuracy (precision)? Then you will never acheive that goal without addressing the known and recorded shortcomings of the design (one of which being the specified acceptable tolerance ranges).