Why does the ar-15 have a barrel extension?

Just a guess but it may be to make it easier for an armorer to do a barrel change. Maybe just easier to manufacture that way.

It's for quick and easy barrel changes without having to go through time consuming head spacing. If the barrel extension is made and installed properly the head space should be GTG. It's just another amazing Stoner design.

The assumption that locking lugs are machined in the barrel is where things are going astray. For the most part the lugs are machined in the receiver and then the barrel is inserted, head space is set, and the barrel locked in place.

With lugs in the barrel, it would require shipping a matched bolt headspaced to fit, and getting that headspacing would require tedious machining of the bolt face to do it. The lugs can't be easily altered to fit the cartridge length.

Germane to the conversation, list the guns with integrally machined locking lugs in the barrel behind the chamber. I don't have an extensive knowledge of firearms but I can't think of any.

The obvious answer would be that the lugs on the ar-15 are too deep, in the sense that the tool has to reach far from the centre of the barrel to be able to cut out the space for the bolt lugs, when the bolt is locked. I'm not sure if this is correct though, and I'd love to understand it better.

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It's not that they're too deep to be done that way, just that it's not the most practical approach-especially for a design that considerably predates CNC machinery.

To make them integral, you'd have relieve behind the lugs, then broach the lugs, then clean up behind them, then reach inside with another cutter to clean up the breech face, then do final chamber reaming, and it would all have to be perfect to be able to not require individual headspacing of each rifle. This would not be difficult with a modern precision mill-turn machining center, but in the 1950s, those didn't exist, so making those cuts would require a specialized machine, and add operations that considerably increase machining time. The two part design makes much more sense with conventional manual machines.

If you examine the AR closely from a machining standpoint, every aspect of it is indicative of the era it came from.

Ease of machining.

The barrel extension is a fairly complicated machining. It's easier and cheaper, to machine it separately and thread it onto an already manufactured barrel. Doing it this way, the machinist has access to both sides in a fairly small space, cutting the outer profile and thread in one operation, maybe on a screw machine, then the lugs in another operation and neither are particularly complicated. If you goon up a part, it's just coming out of bar stock, so it's not that big of a deal.


To machine the locking lugs, feed ramps, etc into a manufactured barrel or barrel blank, you've made life more complicated. You now only have access from one side and if you screw up any operation, you are junking a piece of metal that has a lot of time into it already, ie it's a big deal.

Does that make sense?

Why would they? What advantage does it offer?

axxxel said:

I'm still surprised I haven't found any AR-15 barrel maker that makes barrels with integral lugs.

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Why is this surprising? As already explained, there's no real upside to doing this, as it just makes manufacture more difficult and expensive.

I also think you will find the barrel extension is made from a higher grade of steel then the barrel, and heat treated differently for increased strength.

The steel used to make the barrel isn't up to the job.

rc

eldon519 said:

Why would they? What advantage does it offer?

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As far as I can tell, none whatsoever. The barrel extension design is perfectly functional. And as previously stated, it's an alternative to lugs in the aluminium receiver, which is obviously a bad idea, not lugs in the barrel.

Plenty of other designs have the lugs machined in the barrel.

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Plenty but obviously not all. I have seen it with .22 single shots a lot. My first rifle was a Springfield 120A; the boltway and locking lug recess were machined into the rear of the barrel. I still have a .22 LR Stevens 73 and a .22 Mag Savage 63KM that have locking lug recesses machined directly in the barrel; these only have one locking lug on the bolt head and the barrel slip fits into an alumninum receiver secured by a set screw.

Why does the AR-15 have the locking lug recesses in a seperate extension?

The AR bolt head has multiple lugs. It may be that it is easier to machine multiple locking lug recesses in a seperate barrel extension and then mate it to a barrel than it is to make it as a single piece.

Also, the locking lug recess area is much larger in diameter than the barrel; if it were integral with the barrel, a lot of metal would have to be lathe cut off the barrel blank during manufacture.

Interesting illustrations.

Cutting multiple locking lug recesses in the end of a barrel blank must give a machinist nightmares!

rcmodel said:

I also think you will find the barrel extension is made from a higher grade of steel then the barrel, and heat treated differently for increased strength.

The steel used to make the barrel isn't up to the job.

rc

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The barrel is made from steel designed for optimum performance in a barrel, 4150 or 41V50. (It is not a "lower grade", just a "different grade".)

The extension is made from steel designed for maximum strength, 8620.

The barrel is around 35 to 40 HRc.

The extension is around 55 to 60 HRc.

The yield strength of 4150 barrel steel as finished in a barrel is 104,000 psi.

The yield strength of 8620 case hardened and tempered as in a barrel extension is around 125,000 psi.

The barrel has to be able to withstand temperatures up to 1200 degrees and not loose significant strength, and take this type of cycling through out its life. The barrel extension does not get hot enough to change its temper, so it can maintain its strength.

All 'steel' is not the same. The steel for the M16 front sight base is not the same steel as the steel the bolt carrier and barrel extension, nor it is the same as the steel used in the hammer and trigger. That's why AISI has literally thousands of steel grades, each specialized for different applications....

I'm a machine design engineer, and I can tell ya with pretty good certainty that MachIVshooter hit it on the head...that if the barrel/extension were 1 piece a broaching operation (certainly back then, in mass production) would be required to get the cutter in past the lugs.

(FWIW, headspace check is still necessary!)

Also, FWIW, the M14 receiver and, I believe, the M1 Garand receiver were made of 8620 alloy, mostly for a combo of surface harden-ability and still having core shock resistance. It's also used in a lot on gears, those characteristics also being highly desirable for that application.

One other fact to point out on this, axxxel mentions a few rifles that have the locking lugs, or locking areas of the barrel machined as part of the barrel and not a separate part, what he fails to consider is the cost of the barrels he mentions.

For example a barrel for a Blaser R8, purchased new, is around $1K and that is if you can find it on sale, expect to part with $1500 for many of them. Now they are evidently excellent hammer forged barrels, shoot wonderfully by most all accounts, require no special tools to install or headspace, and are made of excellent steel that is then nitrocarubized after final machining so a long service life can be expected. However for that kind of money you could re-barrel an AR15, with a new bolt 3-4 times with a top quality service grade barrel made from CMV 4150 in a rifling method of your choice in chrome lined or nitrocarburized.

Now I am sure part of the cost difference is German labor costs, and maybe import tariffs. However I'll wager that a lot of it is the cost of actual machine time required to make the very precise machining operations on the barrel where the locking collets on the R8 bolt head will lock up. In order to ensure that every barrel head spaces dead nuts on the tolerances must have to be held insanely tight. That is the cost of a plug and play barrel, whether it is an integral locking lug/recess or even a screw on variety like those used by Desert Tech on their SRS rifles. Even non integral locking lug barrels that have to be easily user changed and high precision are dirty a$$ expensive. Price a factory Accuracy International barrel for example.

We are lucky the AR uses a separate screw on barrel extension, they are effective, reliable, and keep costs down.

I've never seen a .22 rifle with lugs machined into the barrel!

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Neither has anyone else.

I believe he might be mistaken.

Rc

rcmodel said:

I also think you will find the barrel extension is made from a higher grade of steel then the barrel, and heat treated differently for increased strength.

The steel used to make the barrel isn't up to the job.

rc

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That makes sense. Does anyone know what steel is used in AR barrel extensions? 8620, 9310 or Carpenter 158?

So much talk about the steel used for bolts but rarely is the barrel extension mentioned. If a lug breaks off a bolt you can buy a new bolt but if the barrel extension cracks you have a bigger, more expensive fix on your hands. Of course a bolt lug is small and more likely to break than the barrel extension so maybe 8620 is adequate for the extension.

because you can easily set the headspace without further machining, and the bolts dont need fitting

As I stated above, it was to allow for quick barrel change-outs.
Scroll down to "Backround of the Invention"
http://www.patentsencyclopedia.com/app/20160010938
Make the barrel, install the extension and head space, install the barrel as needed.