AR-15; the progeny of genius.

[timmy]

XL: Thanks! Please indulge me just a little longer!

With a bolt gun, the lugs are lapped and the receiver is lapped to ensure that both lugs bear equally on the receiver. I understand that the bolt locks to the barrel extension (which is part of the accuracy aspect. Old John M was doing this on A5s and Remington 8s a long time ago!), but for the 7 locking lugs to mate with their counterparts in the barrel extension, the bolt must sit squarely aligned in a third part, the upper. If I am hearing you right, you are saying that the lock ring holds the barrel in the upper and the bolt slides in the upper with enough precision that the bolt is reliably aligned for accuracy -- is this right?

For shooting with the head erect, you are saying that there is no cheek weld? If so, doesn't that take some getting used to?

[xl_target]

The chamber end of the barrel has threads that the barrel extensions screws onto. There is a hole in the barrel extension throught which an indexing pin is driven. This deforms the threads, keeping the barrel extension from rotating. When you buy a barrel, it will usually come with the barrel extension already installed. This indexing pin fits into a slot in the upper receiver. When machined to spec., you just slide this locating pin into the slot in the upper receiver and then tighten the barrel nut.

This indexing pin also takes care of aligning the gas port in the barrel so it is vertical . This in turn will make the front sight/gasblock vertical. To help align the gas block with the gas port, there are a couple of slots machined into the bottom surface of the barrel. Roll pins go through the gas block and pass throught those slots to hold the gas block on. On some barrels, meant to take a low profile gas block, there are dimples on the bottom of the barrel to set screw the gas block in place.


See barrel installation here

Since the bolt locks into the barrel extension, it and the barrel extension contain the stresses when the rifle is fired. Bolt specifications (Carpenter 158 steel. There are much better grades of steel available today), call for a hardened and shot peened bolt. That is why you can get by with an aluminum upper and lower receiver. There are four raised longitudinal ribs on the bolt carrier. These are the only parts of the bolt carrier that touch the upper receiver. The bolt carrier moves back and forth in the upper receiver and as it moves forward, it causes the extended bolt to run into the barrel extension. As inertia keeps the bolt carrier moving forwards, the back of the bolt is forced back into the bolt carrier. This makes the cam pin move along its curved raceway, causing the bolt to rotate. This then locks the bolt into the barrel extension.

The three lock rings on the bolt are gas rings. They do help center the bolt in the bolt carrier (a little bit) but that is not their primary function.

Many of today's hunting rifle bolt actions have locking lugs that do not bear equally on the receiver. You can blacken the lugs with a marker, close the bolt and often you will find that (maybe) only one lug is making full metal to metal contact. The guns are still reasonably accurate. Of course, if you want benchrest accurace, you will spend a lot of time to ensure even contact of the lugs. In the AR-15, as long as the bolt rotates closed (locking into the barrel extension), it should hold the base of the cartridge tight up against the chamber.

There is a bit of play in the bolt when it sits in the bolt carrier. You can wiggle it a tiny bit when it is fully extended out of the bolt carrier. It is not rigid like the bolt in a bolt action rifle. The only thing holding the bolt into the bolt carrier is the cam pin (which must be free to move in its raceway). If you notice the lugs in the barrel extension are somewhat ramped; so alignment doesn't have to very precise for the bolt to enter it. The bolt doesn't have to be held rigidly centered in the bolt carrier, unlike the lugs in a bolt action rifle. It is usually precise enough. If you look at a close-up photo of the barrel extension, you will notice that the lugs in there are ramped on both sides. I hope this answers your question about bolt alignment?

Backward pressure produced when the cartridge fires, is contained by the bolt being locked up in the barrel extension. Once the gas comes through the gas tube, impinging on the "gas key", it forces the bolt carrier backwards. This causes the cam pin to move its raceway, which in turn rotates the bolt, unlocking it. Once the bolt is fully unlocked the bolt carrier is able to withdraw it from the barrel extension. As the bolt withdraws, the extractor pulls the cartridge out and the ejector in the bolt head kicks it loose of the bolt, sending it out through the ejection port.

Today's machine can hold incredibly tight tolerances. Even the machine tools in the 1950's and 1960's could hold very tight tolerances compared to what is necessary to machine AR-15 parts. $100,000 will buy you a five axis machining center that will easily do any of this stuff and then some. Even a small machine shop can afford those. Some small gun manufacturers whose shops (that I have toured) have just a handful of employees and can afford a few of those machines. That is what I meant when I said that the machining requirements are not onerous.

For shooting with the head erect:
The AR15 stock is placed very high as the bolt carrier recoils partially into the buffer tube to which the stock is attached. In fact, it is inline with the bore. Look at your average bolt action rifle and you will notice how much the stock drops compared to the bore. You can still get a cheek weld on an AR-15, in fact, for good shooting, you must get a decent cheek weld. Look a the video of us bumpfiring and you will see us getting a cheek weld.

My scope is really not that low. If you look at the scope mounts, you will notice how high they are above the bore. The sights are approx 2.5" above the centerline of the bore. What I think you are looking at, is the ocular and objective distance above the handguard and the top of the receiver. The picatinny rail on top of the receiver is raised up a bit. Look at the above barrel installation photo. The handguard is also above the bore.

[xl_target]

Thank you xl_target for the interesting information and videos - the 'bump-fire' stock is so much fun!

CK, We will have to see if we can get one for the next time you come over.

[timmy]

XL (sorry for late reply) Thanks for that explanation of the gas block, including attachment to the barrel, alignment with the gas port, and alignment in the upper.

Regarding multi-locking lug lockup (i.e., > 2), yes, not all of them will touch. One of the big arguments against multi-lug actions like the Weatherby V and Remington 788 was that, if you painted Prussian Blue on the lugs and closed the action, you'd see that not all of them contacted.

Of course, this "argument" didn't mention firing the weapon, because then the Prussian Blue would have shown contact...

But "blueprinting" a bolt gun for accuracy does include lapping the lugs to ensure that both of them contact the receiver equally -- in bench rest sorts of applications, chamber/barrel/bolt/receiver alignment is required to be held to very close tolerances.

Performing this with a multi-lug bolt would be a more complex operation. However, I do know that ARs are set up for high accuracy. Maybe I need to go back and read that some more.

BTW, I notice that some gas tubes are wrapped several times around the barrel -- what is the reason for this?

I confess, looking at this stuff is so intriguing, and now Ruger coming out with one is so tempting... But I still like the milsurps, and haven't yet given up on the idea of getting one of those Turkish Forestry Carbines. Why I need one, who can say?

Thanks for the explanation!

[xl_target]

XL (sorry for late reply) Thanks for that explanation of the gas block, including attachment to the barrel, alignment with the gas port, and alignment in the upper.

Regarding multi-locking lug lockup (i.e., > 2), yes, not all of them will touch. One of the big arguments against multi-lug actions like the Weatherby V and Remington 788 was that, if you painted Prussian Blue on the lugs and closed the action, you'd see that not all of them contacted.

Of course, this "argument" didn't mention firing the weapon, because then the Prussian Blue would have shown contact...

But "blueprinting" a bolt gun for accuracy does include lapping the lugs to ensure that both of them contact the receiver equally -- in bench rest sorts of applications, chamber/barrel/bolt/receiver alignment is required to be held to very close tolerances.

Performing this with a multi-lug bolt would be a more complex operation. However, I do know that ARs are set up for high accuracy. Maybe I need to go back and read that some more.

BTW, I notice that some gas tubes are wrapped several times around the barrel -- what is the reason for this?

I confess, looking at this stuff is so intriguing, and now Ruger coming out with one is so tempting... But I still like the milsurps, and haven't yet given up on the idea of getting one of those Turkish Forestry Carbines. Why I need one, who can say?

Thanks for the explanation!

Tim,
The AR-15 is not just one rifle anymore. It is the result of a system of components that can be put together in various configurations.
In today's market, the AR-15 generally has four different gas systems.
There is the rifle length gas system, the mid length gas system, the carbine gas system and the pistol gas system.

A rifle length (20"+ inch barrel) gas system has its gas port 12" from the receiver.
A mid-length AR 15 (14" to 20" barrel) gas system has its gas port 9" from the receiver.
The carbine length (10" to 18" barrel) gas system has its gas port 7" from the receiver.
A pistol length (10" and shorter barrel) gas system has its gas port 4" from the receiver.

Function and recoil are adjusted by using a heavier or lighter buffer and there are various different springs available.
My upper uses a carbine buffer and spring. A rifle with a rifle length gas system will use a rifle buffer and spring, etc.
There is also dwell time to consider; dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the rearward movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of). A heavier bullet has a longer dwell time. Note that the gas is released into the gas tube only after the bullet has passed the gas port. Your buffer and spring has to be balanced so your AR-15 will cycle with the lightest loading of commercially available ammo or a cartridge with a heavier charge in it. It's amazing that the thing actually works at all. It's equally amazing that the AR-15 actually works so well. As far as accuracy here, the major component of it seems to be the result of having a high quality barrel (which includes a properly cut chamber). That is one of the reasons I bought the upper that I have; with the FN hammer forged barrel.


Here is photo of my rifle's gas system. The standard carbine gas tube looks like this.
The gas block is centered on the gas port which is 7 inches from the receiver.


As far as the gas tube wrapping around the barrel, I think you are talking about this:


Images from http://vtsupply.com/phase-5-ar15-m16-pi ... ar-15.html. Read the text in the link.

When you have a AR15 pistol or a SBR AR15, the gas tube gets very short and this can cause timing problems. Harsh actuation caused by the bolt carrier starting to move backwards, before the pressure in the chamber has decreased, is also an issue. This can cause extractor wear, extractor failure or cartridge rim failure. I suppose it can also result in bolt carrier wear, buffer and spring wear as well as battering of the upper receiver .

Some companies sell adjustable gas blocks. With these adjustable gas blocks, you can restrict the amount of gas entering the gas tube. However, then you have an possible issue of some less powerful commercial ammunition causing the bolt carrier to short cycle.

The wrapping of the gas tube around the barrel lengthens the gas tube in the hope of mitigating some of these issues. Does it work? I have no idea.

[timmy]

XL, the link seems dead, but I found this: http://phase5wsi.com/ar-15-m16-pistol-p ... -tube.html

I can see how this would work, although it's surprising that so little volume under so much pressure would make so great a difference. I confess, one of these does look inviting. Thanks for your patience and explaining!

[MoA]

You cannot shoot surplus military ammo or even ammo loaded commercially to military specs and expect to shoot tiny groups. Granted, some surplus ammo will shoot better than others but you just cannot get handload accuracy with surplus mil. ammo. While some commercially loaded hunting ammo might give you decent groups, they still won't be in the same category as properly tailored handloads. You might also have to tailor your loads to the specific rifle.

1. Military ammo isn't as accurate as many believe.

2. Anyone that says they routinely shoot sub-MOA groups with iron sights and military ammo has a problem with truthfulness.

3. Some military ammo shoots better than others.

4. Military ammo is not "more accurate" when shot from a bench-rest quality rifle.

5. Nothing will match well-developed handloads.

And take one more look at group #8. That's what a fine target AR will do

A: GP-11 is milsurp ammo that is very difficult to replicate with hand loads.
b: No one in their right minds will shoot mil-surp out of a BR rifle. As far as I know.. there are no milsup calibers used in BR.

[xl_target]

MOA!!!
Good to have you back on IFG.
It's been a long time.

Why is GP11 ammo hard to replicate? Do they use a blend of powders?

[xl_target]

I can see how this would work, although it's surprising that so little volume under so much pressure would make so great a difference. I confess, one of these does look inviting. Thanks for your patience and explaining!

Dammit Timmy,
I read your statement and thought; "hmmm, why not".
SInce you asked that question about the pigtailed gas tube, I've spent a lot of time on research and have decided that I want an AR15 pistol.


An AR-15 pistol. The buffer tube sticking out the back makes it longer than usual.
Yes, legally, that is classified as a pistol by the BATF
image from here

I can see a lot of people saying; "why do you want something as big and unwieldy as that"? The answer is to be found is the way the laws are setup.
Many of these laws were setup by politicians who have no clue about guns and shooting. Many of them have have received their gun education diplomas from Hollywood.


In a natural disaster, or a situation involving a riot, I firmly believe that my standard carry gun ( 9mm, .40 S&W, etc) will be just a stopgap measure till I can fight my way to a long gun. In other words, in some situations, a standard handgun just isn't going to be enough. It's not a big deal if you are at home, because you have access to the entire contents of your gun safe. There are times when I am travelling some distance from home or when I am camping/hiking in the middle of nowhere, when it would not be possible to have access to the increased firepower and precision of a rifle. There are also many places, like State Parks, National Parks, etc. where I cannot carry a rifle but I am permitted to carry a pistol under my permit.
My pistol permit allows me to have a pistol loaded and handy when driving. I can have a rifle but in many instances, it is required to be kept in the trunk in a locked case with unloaded magazines. I won't have time to say to a crook; "hold it, hold it, lemme load my mag". So I want a rifle power cartridge with a large capacity magazine that I can carry with me at all times.

[kshitij]

Hi xl,
I have nothing to contribute here. All i can say is that I have been in awe of this thread and have been keenly following it! It gives me a feeling i do not normally get, Jealousy
Damn! The grass is so much more greener on your side
Next time you go out shooting, fire a few mags on our behalf please.
Cheers!

[xl_target]

Hi xl,
Next time you go out shooting, fire a few mags on our behalf please.
Cheers!

Hey kshitij,
Appreciate your comments even if you don't feel you have something to add.

I don't know when I will be going to the rifle range again as the Minnesota winter is upon us now.
I coulda sworn it was just the other day that I was complaining to TC about summer not being here yet.


A few more months of driving in this before we can hit the range.



However, when I do, I will surely pull the trigger a few times for you guys,

[Baljit]

Hahahaha

you should watch this......http://www.accuweather.com/en/ca/kelown ... ther/47170



Baljit

[xl_target]

Wow Baljit, I'm surprised. We are actually colder than you guys.
Usually we "blame' the Candians for sending their cold air masses down on us but this time I can't do that.
Now its warming up and it is supposed to get above freezing this weekend.

Are you reloading like crazy, now that winter is here?

So tell me, have you looked at AR pistols at all. What do you like for a barrel length?
I want something extremely lightweight so I can pack it in my backpack and not have to lug too much weight.
Every ounce counts when you are hiking to the middle of nowhere.

[timmy]

I can see how this would work, although it's surprising that so little volume under so much pressure would make so great a difference. I confess, one of these does look inviting. Thanks for your patience and explaining!

Dammit Timmy,
I read your statement and thought; "hmmm, why not".
SInce you asked that question about the pigtailed gas tube, I've spent a lot of time on research and have decided that I want an AR15 pistol.

XL, a few years back, there was such a pistol for sale with a carbon fiber (or some such material) receiver called (I think) a "Carbon 15." Maybe that Bushmaster company bought them? I thought one would be fun for varmint shooting, like prairie dogs. It could be easily carried and reach out much farther than any other handgun that I own. (Varmint shooting with handguns is a lot of fun in my book!)

I visited my Brother over last weekend and our conversations reminded me of priorities -- i still need to get one of those SMLE 410 muskets. Gas tubes need to wait a little bit. So many guns, so little time!

[xl_target]

Tim,
Yes, Bushmaster bought "Professional Ordinance". There is nothing wrong with the Bushmaster Carbon15 rifles.
The Carbon15 pistols for the most part seem to work. IMO, there is a little bit of a "plastic gun" bias against the Carbon15.
Almost like the "plastic gun" bias against Glocks when they first came out. Now everyone does "polymer".

Here's a video on the Carbon 15 pistol.
The guy seems a little dangerous with his handling of the pistol but the firearm itself seems to be sound.

[youtube][/youtube]