There are a lot of ways to make a barrel, each adherent to each school has strong opinions on each method.
I’m not gonna talk about that any more than I want to talk about the suitability of a specific caliber for battle. This is not that conversation.

Franklin Mann started out doing his tests in what he called a ‘Sled” rest, with the buttstock of the gun placed in a groove, so he could return to more or less the same place every time. Soon enough, Mann discovered that the variables of using a completed rifle made the experiments he was doing more rather than less difficult.

So he made a series of V blocks and used concentric threaded rings on a series of handmade barrels made for him by such famous makers as Pope. Mann had a setup where he shot down a tunnel made of muslin placed on stands, and into a box filled with oiled sawdust, to be able to carefully capture the trajectory of the bullets, (he placed cardstock every few feet) and the bullets. The mechanism he made to do all this testing shows not that he was an eccentric, but plainly and probably a lunatic. Here’s a picture of the shooting rest and “tunnel” and here’s a picture of some of the barrels he used to test, including some of the knurled chamber/firing mechanism segments.

Mann realized that in order to test only the barrel and the bullet, you had to isolate them from everything else. And that’s what he did, and his results speak for themselves. If you haven’t read the book, you might consider it.

Shooters from then on have understood that there are a bunch of things that can affect accuracy. Pope made a pretty comfortable living tuning single shot rifles with gain twist barrels. Everyone who has ever shot a bolt action knows that as the barrel warms or of the stock absorbs some moisture it can affect the point of impact.

Volumes have been written about free floating barrels, pillar bedding actions. And yet there are still lots of good shooting Mannlicher-Schoenauer rifles out there. Other volumes have been written about barrel harmonics and damping systems and god knows what have you.

Here is the very bottomline: For each barrel out there, there is an optimum load or a series of optimum loads, an optimum round that fits the barrel’s chamber as well as it can. If you place that barrel in a machine rest, and insert a series of optimum loads and fire them at distance X, you will end up with an accuracy that can be measured. Whatever that MOA value is, that is all that it will ever be.

I’m not talking about firelapping or any other methods of accurizing, this isn’t that conversation either. Under controlled circumstances, you will get what you get out of a barrel, and you will never get any more.

In a boltie you can tweak the stock, bed the action, bed the barrel, float the barrel, cork the stock, lap the lugs, any number of things to try to wring the most accuracy out of the barrel. A falling block has a different set of accuracy related components. Break action rifles another set. Doubles- well, doubles aren’t meant for long ranges anyway.

In an AR all of these things have been removed from the process. Installed correctly, the gas tube touches nothing, and cannot exert any kind of force on the barrel. (I can’t speak for piston types) Installed correctly, and manufactured correctly, the front sight cannot exert any kind of force on the barrel. The handguard can only exert a relatively small bit of tension, and there are even “Free floating” handguards.

This is one of the aspects of the design that reveals it’s genius to me. The barrel is held to the receiver by a nut. Nothing else touches it in a way that can affect the barrel’s accuracy. It hearkens back to Mann and his separation of the barrel from all potential external influence.

I see a lot of guys messing with AR’s in their basement shops on youtube. The recurrent themes seem to involve things like squaring the receivers. There is nothing that can be done to an AR receiver that will make the barrel shoot better than the barrel will shoot. If you think this is possible, beware of hucksters selling oceanfront property in Florida. The accuracy of the barrel is a fixed quantity, and messing with the receiver cannot change that.

I’ve heard the argument that “Squaring the receiver aligns the bolt properly so the headspace is perfect” &etc.

Another fun feature of the AR is that the bolt has a bit of play in the bolt carrier. Not a lot, but if you pull the BCG out of your rifle you will discover that the bolt will wiggle a bit in the bolt carrier. This is not a flaw, it is a feature. The bolt itself is designed so it can compensate for the maximum misalignment that can theoretically occur between the receiver and the barrel. So if the barrel is not square to the receiver by .05 of a degree, the bolt will still find the cutouts in the barrel extension and lock up against the cartrige, in it’s chamber.

Oh, about the chamber. Ed Foster send me this missive, and regardless of what you think of me, you should pay attention to Ed. I may know a lot of shit about manufacture, but Ed knows from AR’s. I quote:

Chambering is simple. The MilSpec 5.56mm and the civilian .223 Rem both have identical chambers.

The difference between the civilian and military barrels is in front of the chamber. The .223 has .025 freebore @ .225 diameter and a 1.5 degree leade cone into the rifling.

The military tube has .060 freebore @.226 diameter and a 3 degree leade into the rifling, to accomodate the much longer ogive on the tracer round. The extra long tracer projectile is also the only reason the military uses a 1 in 7 twist. The 1 in 8 works better with both the 62 grain green tip and the Mk. 262 77 grain load. Better accuracy with the 62, and lower pressures with the 77.

The “compromise” chamber I’m most familiar with is the modified Wilde chamber found on Roy Piontek’s E.R. Shaw barrels, a just about “min” chamber with .032 length freebore x .225 diameter, and it shoots like a dream with everything I’ve put down it. It maintains the military 3 degree leade angle, and that seems to have been the critical pressure control mechanism. I concede it might cause some problems if fired continuously with tracer, a hot loaded and brutally dirty round, but what sane person shoots tracer in a good barrel anyway?

I’ve received the exact same comments on the Wilde chamber from Jim, and it appears to be the perfect advice.

As we’ve already discussed, the bolt, barrel, and barrel extension make the accuracy of the barrel. I have not tried this, but I have a solid feeling that if I were to take those three parts to the range, put the barrel into a suitable vise, chamber a round and put in the bolt, then tap the firing pin, it would shoot just fine. (I don’t imagine it would do the components any good, though I can’t see as it would do them a good deal of harm, either)

So the receiver is just a sort of a carrier for the barrel, with an area for the bcg to reside.

If the barrel extension doesn’t have parallel surfaces on the mounting ring, that could cause the barrel to be misaligned with the receiver.
If the barrel extension isn’t internally square to the barrel, that could cause a different type of misalignment.
If the front of the receiver isn’t square to the baore, that can cause yet another type of misalignment. And if the internal threads of the nut are not square with the seating surface, this can cause misalignment or looseness. It seems that these are the sort of problems that all the home gunsmiths I see online are attempting to correct.

Why on earth would you want an improperly manufactured part in your firearm? Those potential problems are manufacturing defects, and the best thing to do with parts that are improperly manufactured is to send them back to their manufacturers with some explicit directions as to their placement. You cannot make a silk purse out of a sows ear.

In any event, the bottomline is still the barrel, extension, and bolt. Those pieces together provide the accuracy, the rest of it just carries them around and makes them convenient to use.

Here’s the thing: Having a loose or improper fitting barrel/receiver interface will cause the sights to move or not line up properly or repeatably. This will not allow you to use the accuracy that the manufacturer put into the barrel. At Prof Hale’s place I was assaulted by people who just KNEW that fucking with the receiver was going to make the barrel shoot more accurately. This is voodoo gunsmithing, pure and simple, and to represent it as anything else is bullshit.

Now, I will gladly concede that making that receiver fit the barrel tightly will make sure the sight can be made to point at the thing the barrel can hit much more reliably, but again, why deliberately use an out-of-tolerance part, if max accuracy is your goal?

The way the barrel fits into the receiver is critical, so those dimensions have a tighter tolerance. The slot at the top of the receiver is critical for bolt alignment. The depth of it has to allow room for the receiver to grow. The nut is steel so that it expands at the same thermal rate as the barrel, so it doesn’t cause the threads in the receiver to lock up. This is not because it can affect the accuracy of the barrel, which it cannot, but because it affects the tightness and repeatability between the receiver (Which is where the rear sight or optics are mounted) and the barrel. If the optics are not in a rigid and immobile relationship to the barrel, there is no way you will shoot accurately.

This is all some pretty damned fine engineering, but there are a lot of things you can’t even see. Mr Foster points out something I knew but maybe you don’t: Flip the upper up on it’s forward pin. See the hole for the back pin? It’s a slot. It’s not round. It’s just a little bit longer than it is tall, and the reason is to prevent the upper from binding when the receiver warms while being shot.

If you have a digital IR thermometer, you can pump a few dozen rounds quickly through your AR, and note the temperature change, and measure the diameter of the pin and the length of the slot, and use that to determine what alloy the upper is probably made from. Congrats, you just reverse engineered that part of the design.

Additionally, and i don’t know if all uppers are like this, mine and several others I’ve seen have had the slot canted at a slight angle. This means as the upper warms, it fits a little more tightly to the lower. That slot is a little piece of brilliance there, it really is.

I’m not concerned about the suitability of this as a battle rifle, I’m really, really not. That discussion is for other people. I’m talking about the incredibly complex and well thought out design of what in the end is a very simple mechanism, and one that is not just elegant but a blast to shoot. Thanks, Gene. Nicely done.