Neanderpundit

The kit I bought from Sarco had everything I needed, but the hammer was questionable. Prof. Hale gave me one (and some other parts) from his personal stash; it went in perfectly and worked perfectly- having seen both hammers, I now know mine had missed a manufacturing step, hence it’s failure to operate properly.

Aside from that, the lower went together with remarkable ease, and yes, you could easily put it together on a TV tray. Mine will most likely remain an M4gery it’s whole life, though I kinda like the look of that Magpul UBR stock. I’ve always been a sucker for secret compartments, and having a place t store a wound up pullthrough could be of some utility. I see they make a plug so you can use the pistol grip similarly, so I may go that route instead of the $250 worth of magpul plastic.

I have approached this whole subject through the eyes of a mechaincal geek, and all the things I’ve discussed have related to how the firearm is designed and how it works and how it is supposed to work. Anyone who has ever worked on one, who is not a fool, can see the really marvelous thought that has gone into it’s design, and the brilliance that is in the basic manufacturing process.

Having never carried one in battle, and having only put a very few rounds through mine (though plenty through other’s) I can not make any claims as to it’s effectiveness as a battle rifle, and I won’t, and I won’t even have that discussion.

Having shot them in their many forms, I can say, it’s a hoot to shoot, and I look very much forward to shooting mine more. The time may come that I do another, after all the pieces are inexpensive enough. Putting my hands on a really tight heavy barrel and a freefloatinhg handguard? Could be awhole lot of fun. But then I don’t have too many places I can shoot that far anyway.

All in all, from a pure engineering standpoint, I am impressed. And I don’t impress easily.

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.

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.