Prof Hale asks in comments at the previous post:

“What about the differences in expansion of the materials? Where the barrel meets the receiver, steel on aluminum. Cold, you have a tight fit. But as you warm it up does the receiver expand faster than the steel barrel and thus produce slop (non-repeatable) barrel alignment? “

Well, no. That’s not much of an issue, an again, it’s because of the design of the rifle.

The barrel orients on a pin that fits in a corresponding slot in the receiver. The barrel is held on by a nut. the nut is steel, the barrel is steel. When you tighten the nut, you make sure the seating surfaces on the receiver, on the barrel extension, and inside the nut are clean and lubricated. Then you run the nut in until it makes contact (This is a specific concern I had, because I tend to overtighten and I didn’t want to damage anything) and then tighten until the next whole groove(or hole) comes into line.

The nut I had had 20 grooves. The thread pitch is 18 tpi. If I tighten one whole groove, the amount of “preload” with which I have wrung the barrel into the receiver is .0027″ or just under three thou of “pre tension” in that barrel/receiver/nut combination. If I only go a “half groove” it is of course half of that.

Steel has a coefficient of thermal expansion of 0.000016 meters per meter per degree centigrade. Aluminum has a coefficient of thermal expansion of 0.000023 meters per meter per degree centigrade.

Let’s say that the barrel/receiver combination warms from 70 degrees (Room temperature) to 250 degrees during shooting. And let’s say that the barrel nut engages 1/2″ of thread. That 1/2″ of thread, over that temperature range, will allow that 1/2″ to grow .0011″.

That could seem like a lot, except that during that same time, the barrel nut warms too, and it will grow .0007″ over the same length of thread. (if it didn’t, the threads wouldn’t fit anymore!) So the actual difference in the warm and cold fit is in the neighborhood of .0004″.

IN a barrel we have already established has .0027″ of lockup. So No, Prof. hale, the warmth of the barrel/receiver combo will 1: only tighten the interface, and 2: not change temperature enough to be an issue.

And then there’s 3: The barrel and barrel nut and barrel adapter are steel alloy. The receiver is aluminum. The receiver is a natural heat sink, and will draw heat away from the barrel/receiver interface, so the actual amount of difference in size will be much, much smaller.

one thing I haven’t discussed much in this is the barrel adapter, but the same expansion issues mentioned further support what I’m saying.

Also, I’m doing the math off the top of my head, so I may have erred a bit on the calculations, so anyone who wants to correct them can feel free to do so. At the end of the day, the bottomline is the thermal expansion that is possible is always less than the force with which the barrel is wrung in, and the expansion always works to tighten rather than loosen the barrel. if you have a barrel that “Shoots lose” it’s because it wasn’t properly tightened to begin with.

This is all based on there being a strict linear progression of the expansion, of course; remember that the difference between the front surface of the receiver and the very first thread is minuscule, a tiny bit more for the second, and so forth. The magic of the design is that it’s all done there, and not attached by bolts that go all the way through the receiver, for instance, and have to tsake into account the difference in thermal expansion of the whole deal.

I’m forever harping on the use of cast aluminum heads on cast iron cylinder blocks, and this is because of the difference betwen the thermal expansion characteristics of the dissimilar metals. In time, they scrub the gaskets off the heads by warming and cooling, it works like rolling dough betwen your palms, move one hand more than the other and eventually the dough ends up where you don’t want it.

Stoner used this to his advantage in the AR; he knew that when the aluminum warmed it would tighten everything up, but not enough to be an issue.

Now, let’s say that your nut has a burr inside so the surface of the inside of the nut is not perpendicular to the centerline of the bore. Or the face of the receiver isn’t perpendicular to the bore, or the surfaces of the barrel adapter are not parallel to one another or perpendicular to the threads.

This is where the basement gunsmiths are saying ‘Oh, I can accurize that by doing X, or Y, or “Truing the receiver” etc.

I deleted what my original opinion was of anyone who would buy substandard parts and then expect them to shoot tiny MOA groups. All I am going to say is, why, if your aim is to have a precision rifle that shoots very accurately, why would you build it out of sows ears? If you need to fuck with the parts on an AR to make it “Right”, then you bought the wrong parts.

Finally: when you have gotten all the pieces put together, you wil never, ever, ever get more accuracy out of the barrel than the barrel by itself brings to the equation. There is nothing you can do to make the barrel more accurate the doesn’t involve modifying the barrel in some way. Nothing that can be done to the receiver, upper or lower, will change this.

All that you can hope to do is try to improve the fit in the interface between the barrel, the barrel extension, and the receiver. And all this will do is make the relationship between the optics/sights and the barrel more fixed.

let’s say you have a barrel with 1/2 MOA as measured at 200 yards.

let’s further say you have a receiver with a non true face.

If you tighten the barrel so it’s tight, and cannot move, the sight will be out of line with the barrel.

Through a magical, secret process we like to call “Sighting in”, you adjust the sights so that they point at the point of impact of the rifle.

The MOA does not change. having a rifle properly sighted in will allow you to shoot better, but the barrel can and will only give you what it has to give, period.

Nothing. Anywhere. Ever. That. Can. be. Done. To. An AR’s Receiver. Will. Improve. The Accuracy. Of. the barrel.

Got it? No? well, then you’re beyond my help.