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[ARCHIVED THREAD] - how light can I go on an AR 10 barrel / dimpling?

Posted: 4/5/2015 2:55:29 AM EDT

I have a AR10 SBR, I think I'm gonna keep it, I just need to lighten it up. The barrel a straight .750 from the chamber to muzzle, 12.5" long. Its too heavy. I want to cut weight, particularly out front. I'm not concerned about acuracy really, I'd be using it as a brush gun for hogs as it would be too heavy w/ scope anyway, just an upclose blaster for harder targets.

I mostly want to be able to carry it around & have better balance. I would get a new lighter handguard & do some other mods to the recivers & BC to cut weight but the barrel is the place to start after the handguard.

My question is just how light can I profile it? Or perhaps I should look into ball dimpling?? Is it possible a ball milled barrel can be as light as a barrel contured to minimum profile or is the OD of a ball dimpled barrel always larger than the OD of a pencil barrel?

I've got about 2" in front of the gas bloc, I can take that from .750 to .600. That is the same OD as the new G2 AP4 barrel ahead of the carbine position gas block. The armalite pencil carbine middy barrels are .580" from the gas journal to the muzzle, maybe I could go that skinny but my gas block is about an inch closer to the reciever than a middy.

Behind the gas block I have about 4.5" to the chamber taper. Per my notes, an Armalite carbine pencil barrel I had was .750 tapered down to .680" behind the middy gas journal shoulder. My gas block is leonidas, shorter than a middy so I figure that I should be able to go to .700 at that taper rate. A 4.5" taper from .750-.700, that's not going to make much difference? Can they cut the ball dimples deeper that the min OD of a pincil type barrel.

Weren't the original M14 barrels like .650 under the hand guards??

ETA: I got the upper apart. more info on pg 2, but it looks like the chamber area of this barrel is way too much, compared to a m14 barrel anyway...

Posted: 4/5/2015 2:22:35 PM EDT

[#1]

I talked with ADCO about reducing weight on a 308 bbl once, their answer was turn it down vs fluting. Never asked about dimpling. They did say there was no way to say how accuracy would be affected.

Posted: 4/5/2015 3:33:34 PM EDT

[#2]

Just to throw this out there. The barrels on many of the early AR-10s were at least .600" or thinner in front of the FSB, especially the Cuban models. The standard barrel on a FAL is .590" in front of the gas block and I'm running a Rainer barrel that was turned down to .625 in front of the FSB on my Cuban clone - no problems so far. In fact I think I should've had it turned down to .600 or .590 to begin with. It still looks a bit too thick. Oh, and as far as accuracy goes, the thinner barrel doesn't seem to have affected things one little bit. Just my two cents worth. Regardless, you should confer with the barrel manufacturer as far as safe limits in turning down a barrel.

Posted: 4/5/2015 3:58:43 PM EDT

[#3]

I have a Cuban AR10 barrel infront of me now and the diameter is 0.520" in front of the FSB.
Its 0.642" under the FSB.
And the threaded muzzle is 0.510"

Posted: 4/5/2015 4:31:11 PM EDT

[#4]

Quoted:

Weren't the original M14 barrels like .650 under the hand guards??

View Quote

The M14 barrel is a pretty complicated shape, you can find the drawing online if you look hard enough.

The swell for the chamber would equate to being 1.20" diameter for 1.25" from the front of the barrel extension to a 12 degree included taper to a .745 diameter from the 0.745" diameter it tapered to 0.627 just aft of the gas port. The portion of the barrel ahead of the gas port was a cylindrical 0.595" diameter

If the barrel is 4140 or 4150 you can probably go to these dimensions without loss of strength. If you have a 416 stainless barrel I would increase the dimension by about 30% just to be safe.

You can really remove some weight on a .308 AR type rifle if you use that profile.

Quoted:
Oh, and as far as accuracy goes, the thinner barrel doesn't seem to have affected things one little bit.

View Quote

Since there is very little hanging on the end of the barrel (unlike the M14 or FAL) the light profile works better.

Posted: 4/5/2015 11:29:55 PM EDT

[#5]

Thank you so much to the guys who have contributed pics & info.

So the M14 barrel tapered from .745 to .627... I need to see how long gas system of the M14 was....
Found a pic:

The gas port is over 13" from the breech. Seems like it is .700 7.7" from the breech.
http://photos.imageevent.com/badgerdog/generalstorage/m14barreldrawings/M14%20Barrel%20Drawing%201.pdf

It must be the original FAL barrel that was skinny under the HG. I'll have to look for those specs. Here is a pic of a parts kit:

My gas block is exactly 8.5" from the upper receiver, that is only like 1/2" less than middy I guess, and I do not have a gas block shoulder so I guess I will just have it tapered from the .750 at the chamber to .685 prior to the short shoulder. After the gas block I will make it .580 to .510 if he will let me.

Posted: 4/6/2015 12:01:52 AM EDT

[#6]

Quote History

Quoted:
I have a Cuban AR10 barrel infront of me now and the diameter is 0.520" in front of the FSB.
Its 0.642" under the FSB.
And the threaded muzzle is 0.510"

http://oi48.tinypic.com/11t4f87.jpg

View Quote

So how does a guy obtain original pattern cuban AR10s?

Thanks again for posting the pics & measurements.

Posted: 4/6/2015 9:29:45 PM EDT

[#7]

The Cuban and Sudanese AR10's were made in Netherlands from 1957 to 1959 or so and those parts
are completely incompatible with modern AR10's (including clones and AR308's). The barrel photos and
dimensions were given for reference as to possibility for lightweighting the barrel. If you can find a
gunsmith willing to custom flute a barrel I would be willing to provide more detailed
measurements.

See photos below of Cuban AR10's and note the caption on the first photo that claims a 6.85 lb weight.
The Cuban rifles did not have the heavy bayonet lug and were indeed less than 7 lbs.

Posted: 4/6/2015 10:28:20 PM EDT

[#8]

HHollow,

What's the diameter and length of those real AR10 barrel extensions you have? By length, I mean length from the rear to the extension flange. I want to compare them to a modern extension for reference.

Posted: 4/6/2015 11:12:49 PM EDT

[#9]

Dia = 1.200"
Len = 1.242"

I can guess what you are thinking - the bolt lug pattern is way different.

Posted: 4/6/2015 11:22:03 PM EDT

[#10]

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Dia = 1.200"
Len = 1.242"

I can guess what you are thinking - the bolt lug pattern is way different.

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I only care about the barrel extension tunnel geometry for reference purposes.

I just measured a modern extension...

Dia = 1.195"
Len = 1.235"

Posted: 4/6/2015 11:29:00 PM EDT

[#11]

The original parts are very gard to come by and expensive when found. There is absolutely no reason to use original pre-1960 parts on a newer rifle.

However, since there is a shortage of the original parts some have thought about makeing them work on original rifles. CarbineKid has actually fitted a modern barrel to a Dutch AR10 by slightly shimming the dia of the extension. With some more fiddling he was also able to make the modern bolt work with the Dutch carrier. I think with some tinkering it could be done very well.

Posted: 4/7/2015 4:12:19 AM EDT

[#12]

yes, I undertand that the original ar10s are not compatible w/ the new ar10s etc.

I would enjoy having an old one as a collectable I guess.

Holy shit, you are telling me that a gun w/ a barrel as pictured in your earlier posts w/ the full fluting under the handguard only weiged 6.85 lbs? Sure that barrel is fluted but it seems like a lot of metal.

Anyway, if you don't mind sharing the minimum diameter under the hand guard of those fluted barrels for reference that would be nice but I will just have the barrel turned down as skinny as possible w/o flutes or dimpling. The gun would not be for gun fights and for up close so I'm not concerned about it getting hot.

Posted: 4/7/2015 7:17:39 AM EDT

[#13]

Quote History

Quoted:
yes, I undertand that the original ar10s are not compatible w/ the new ar10s etc.

I would enjoy having an old one as a collectable I guess.

Holy shit, you are telling me that a gun w/ a barrel as pictured in your earlier posts w/ the full fluting under the handguard only weiged 6.85 lbs? Sure that barrel is fluted but it seems like a lot of metal.

Anyway, if you don't mind sharing the minimum diameter under the hand guard of those fluted barrels for reference that would be nice but I will just have the barrel turned down as skinny as possible w/o flutes or dimpling. The gun would not be for gun fights and for up close so I'm not concerned about it getting hot.

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I think the really light ones had really light barrels:

Here is a picture from OlGunner posted on this thread "https://www.ar15.com/archive/topic.html?b=3&f=123&t=553409" showing a barrel profile that is not much beefier that a USGI M14 barrel.

I have a feeling that the heavy barrel of the later ones was done to address the full-auto barrel heating issues that would plague the AR15, (and M14)

Posted: 4/7/2015 2:06:05 PM EDT

[#14]

The inner diameter of the fluted barrel can not be measured by my simple caliper. I will see what I can find to do the job.

In the mean time, the earlier AR10's (Cuban, Sudanses, Guatemalan) were light all around, from the barrel to the bolt. The latest model (1960-1961) were heavy all around as is evident by the bolts shown below. The earlier bolts had the smaller lugs, the later bolts had the heavier lugs. The bolts in the earlier AR10's were reputed to fail at 4000 rounds or so. I have never seen a large lugged bolt break.

Posted: 4/7/2015 6:08:48 PM EDT

[#15]

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The earlier bolts had the smaller lugs, the later bolts had the heavier lugs. The bolts in the earlier AR10's were reputed to fail at 4000 rounds or so. I have never seen a large lugged bolt break.

http://oi59.tinypic.com/hs87tg.jpg

http://oi42.tinypic.com/2ijt72u.jpg

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I can see why it failed right at the middle lug, they should have moved the ejector towards the center.

Posted: 4/7/2015 9:47:00 PM EDT

[#16]

Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

Posted: 4/7/2015 11:31:56 PM EDT

[#17]

HHollow,

What are the dimensions of the reciever?

Can you compare the reciever dimensions to the of an AR-15 or a SR-25 pattern 308 AR?

Posted: 4/8/2015 12:29:11 AM EDT

[#18]

The original lower receivers are only vaguely similar to modern AR10, SR25, and clones.
The mating profile between the upper/lower are unique
The buffer tube is a substantially greater diameter on the early receivers
The pivot pin and locking pin size and positions are different
The hammer/trigger pin holes are a larger size and in unique locations

No parts interchange. However, it is reputed that CarbineKid has demonstrated that the original lowers can be heavily modified to use modern AR15 trigger groups.

Posted: 4/8/2015 6:10:18 AM EDT

[#19]

so is there a way for a non SOT/rich to own these are are they all pre 86 transferrables?

Posted: 4/8/2015 6:12:13 AM EDT

[#20]

Quote History

Quoted:
Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

View Quote

I really appreciate you checking into that.

So I guess that I am still looking at .750 tapering to .685 as a min diameter?

Posted: 4/8/2015 10:19:14 AM EDT

[#21]

Quote History

Quoted:

I really appreciate you checking into that.

So I guess that I am still looking at .750 tapering to .685 as a min diameter?

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Quoted:

Quoted:
Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

I really appreciate you checking into that.

So I guess that I am still looking at .750 tapering to .685 as a min diameter?

The barrel at the muzzle threads will be .625" (5/8"x24 thread). Given your barrel is short, I would go for as light as possible under the handguard, and lean out from the gas block forward. By that I mean; cut the barrel diameter small right up to the gas block location.
SLR makes a titanium adjustable gas block. This will help to minimize weight on the barrel.

I'd look at a slightly heavier buttstock to shift felt weight towards the magwell and balance the rifle better.

Posted: 4/8/2015 12:30:16 PM EDT

[#22]

Quote History

Quoted:
Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

View Quote

Flutes add stiffness, not strength. Hoop stress is the cause of failure. Since the flutes are not connected radially, excuse me - circumferentially, they cannot add to the hoop strength. The maximum strength will be governed by the hoop stress at the minimum diameter that completely surrounds the bore.

If you do the stress calculations for a thick walled cylinder, a 4150 barrel could be turned down to about 0.52" and still contain maximum pressure (55,000) with no safety factor. But, since the maximum pressure the barrel sees is dependent of the distance from the chamber, the thickness 5 inches from the chamber can be considerably thinner.

Without talking to an experienced barrel manufacturer, I would say the minimum profile for a .308 Win barrel would be no less than a known barrel. And an M14 is pretty light for a .308.

Posted: 4/8/2015 2:53:20 PM EDT

[#23]

I wish someone would make an AR-10 on the original pattern, but with a flat top (and T-handle or side charger). I love how light and quick-handling they are, even with a 20" barrel, compared to the overweight modern versions with too long of a LOP.

Thanks for the awesome pics and info on the "real" AR-10. :)

Posted: 4/8/2015 2:59:53 PM EDT

[#24]

Quote History

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The barrel at the muzzle threads will be .625" (5/8"x24 thread). Given your barrel is short, I would go for as light as possible under the handguard, and lean out from the gas block forward. By that I mean; cut the barrel diameter small right up to the gas block location.
SLR makes a titanium adjustable gas block. This will help to minimize weight on the barrel.

I'd look at a slightly heavier buttstock to shift felt weight towards the magwell and balance the rifle better.

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Quoted:

Quoted:
Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

I really appreciate you checking into that.

So I guess that I am still looking at .750 tapering to .685 as a min diameter?

The barrel at the muzzle threads will be .625" (5/8"x24 thread). Given your barrel is short, I would go for as light as possible under the handguard, and lean out from the gas block forward. By that I mean; cut the barrel diameter small right up to the gas block location.
SLR makes a titanium adjustable gas block. This will help to minimize weight on the barrel.

I'd look at a slightly heavier buttstock to shift felt weight towards the magwell and balance the rifle better.

I did not realize that SLR made a Ti selectable block! Thx. (ETA: they do not. V7 makes a non adj Ti block, SLR makes light steel blocks at 1.3 oz)

I heavy butt can definitely improve balance, but i am most concerned w/ being able to carry it so I'm going to go w/ an extra light stock. Maybe I can rig up a way to add weight to the butt if I am in a static position and I want better off hand balance.

Posted: 4/8/2015 3:11:36 PM EDT

[#25]

Quote History

Quoted:

Flutes add stiffness, not strength. Hoop stress is the cause of failure. Since the flutes are not connected radially, excuse me - circumferentially, they cannot add to the hoop strength. The maximum strength will be governed by the hoop stress at the minimum diameter that completely surrounds the bore.

If you do the stress calculations for a thick walled cylinder, a 4150 barrel could be turned down to about 0.52" and still contain maximum pressure (55,000) with no safety factor. But, since the maximum pressure the barrel sees is dependent of the distance from the chamber, the thickness 5 inches from the chamber can be considerably thinner.

Without talking to an experienced barrel manufacturer, I would say the minimum profile for a .308 Win barrel would be no less than a known barrel. And an M14 is pretty light for a .308.

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Quote History

Quoted:

Quoted:
Here are the measurements:
The inner diameter (under the flutes) is constant down the barrel and is measured at 0.638 - 0.644"
The outer diameter varies, at the start of fluting it is 1.005" and nearest the FSB it is 0.8775"

I understand that the flutes themselves add stremgth to the barrel and that the inner diameter under the
fluting should not be taken as the minimum safe diameter for an unfluted barrel.

Flutes add stiffness, not strength. Hoop stress is the cause of failure. Since the flutes are not connected radially, excuse me - circumferentially, they cannot add to the hoop strength. The maximum strength will be governed by the hoop stress at the minimum diameter that completely surrounds the bore.

If you do the stress calculations for a thick walled cylinder, a 4150 barrel could be turned down to about 0.52" and still contain maximum pressure (55,000) with no safety factor. But, since the maximum pressure the barrel sees is dependent of the distance from the chamber, the thickness 5 inches from the chamber can be considerably thinner.

Without talking to an experienced barrel manufacturer, I would say the minimum profile for a .308 Win barrel would be no less than a known barrel. And an M14 is pretty light for a .308.

Well, ahead of the chamber the GI M14 barrel starts at .745. That is the lightest I know of at the throat, unless you consider the min OD that fluted cuban barrel.

I think max pressure for 308 is 62k?? What is typical safety factor build into gun barrels & bolts?
here is a 308 pressure chart:

at 7" bullet travel pressure 30k. Not sure if that is 7" from the breech or 7" from the case mouth.

The ligest barrel I know of prior to where my gas bloc is is the DPMS GII barrel which goes to .600 ahead of its gas block. I doubt anyone would be willing to taper my barrel from .745 down to .600 behind my gas block, though it shoudl be safe since pressures drop so much when a bullet is that far, but there arent' any known barrels w/ that much taper that fast.

Posted: 4/8/2015 5:51:58 PM EDT

[#26]

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Quoted:
so is there a way for a non SOT/rich to own these are are they all pre 86 transferrables?

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If you lurk it is possible to find a kit-made semi-auto selling on gunbroker every few months. The heavier
Portuguese AR10's seem to be way more common (and less costly) than the ligher earlier models.

There are very few pre86 transferable AR10 MG's that have the original Dutch lower. Most transferables
were converted from semis before 1986. The pre-may sample guns are the most common AR10 MG's and they
tend to be the ligher weight AR10's surplussed from the Guatemalan miltary training academy via Armex.

Posted: 4/9/2015 9:19:58 AM EDT

[#27]

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Well, ahead of the chamber the GI M14 barrel starts at .745. That is the lightest I know of at the throat, unless you consider the min OD that fluted cuban barrel.

I think max pressure for 308 is 62k?? What is typical safety factor build into gun barrels & bolts?
here is a 308 pressure chart:
http://www.realguns.com/images/pvtstdr308hl.gif
at 7" bullet travel pressure 30k. Not sure if that is 7" from the breech or 7" from the case mouth.

The ligest barrel I know of prior to where my gas bloc is is the DPMS GII barrel which goes to .600 ahead of its gas block. I doubt anyone would be willing to taper my barrel from .745 down to .600 behind my gas block, though it shoudl be safe since pressures drop so much when a bullet is that far, but there arent' any known barrels w/ that much taper that fast.

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The graph is "projectile travel", so "zero" is wherever the base of the bullet is in relation to the back of the case.

Actually, if you look at the drawings for an M14 barrel, the chamber swell, which has an effective diameter of 0.900", starts to taper down before the origin of the rifling. And, about 5/8 inch in front of the chamber throat the barrel diameter is only 0.804. And, at 2 inches in front of the throat, the 0.745 diameter starts.

It looks like the designers of that barrel, used a safety factor of 1.5 and proceeded to make the minimum diameter that would contain a proof load.

(You know, once you get out to about 7 to 8 inches from the breech, the barrel wall thickness only needs to be .080", and at the muzzle, .020", and still maintain a 1.5 safety factor, with 41V50)

Posted: 4/10/2015 1:49:29 AM EDT

[#28]

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(You know, once you get out to about 7 to 8 inches from the breech, the barrel wall thickness only needs to be .080", and at the muzzle, .020", and still maintain a 1.5 safety factor, with 41V50)

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at 308 win pressure or 7.62 nato pressure?

Posted: 4/10/2015 7:10:58 AM EDT

[#29]

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at 308 win pressure or 7.62 nato pressure?

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Quoted:

(You know, once you get out to about 7 to 8 inches from the breech, the barrel wall thickness only needs to be .080", and at the muzzle, .020", and still maintain a 1.5 safety factor, with 41V50)

at 308 win pressure or 7.62 nato pressure?

based on the M80 Ball pressure curve.

But, it really doesn't matter that much. The pressure at that point in the barrel is about 30,000 psi for M80, the graph above shows the same pressure at 7 inches, most loads intended for semi automatics will be the same at this point, plus or minus 10% to 20%. Use of a slow powder like 4320 will change the shape of the P-X and P-T curves more than just a change in peak pressure.

(Oh, and Cartridge, 7.62mm NATO, M80 Ball specification states that the maximum average chamber pressure shall be 52,940 psi, and the average plus 3 standard deviations shall not exceed 58,016 psi when measured by transducer. The port pressure [I think this is measured at 12 inches of bullet travel] shall be more that 8,702 psi but less than 12,238 psi. The actual pressures are specified in MPa, which is why they aren't round numbers. Commercial ammunition loaded to C.I.P. standards has a maximum chamber pressure of 60,191 psi, measured by transducer, and proofed to 75,275 psi. So, commercial stuff is ever so slightly hotter.)

Posted: 4/10/2015 12:14:34 PM EDT

[#30]

right, so if a 308 bullet base is about 1.75" from the breech, per that chart i posted on pg 1, after about 6.5" of bullet travel, the beginning of my gas journal, pressures coudl still be up at maybe 32,500 psi, lets say 33k. What would be a 1.5 safety factor for that? Or for 35k psi? Can I get the formula for that?

Armalite commercial steel woudl be 4140 right? 4150 is the mil spec?

The .600 diameter of the G2 barrel starts in front of the gas journal. The gas journal is long, for the FSB, so about 2", so in reality, ther barrel doesn't hit .600 till about 9.5" from the upprer receiver I reckon. Though my gas system is longer, the front of my gas journal is in about the same place since the gas journal is longer.

Posted: 4/10/2015 2:39:56 PM EDT

[#31]

Quote History

Quoted:

The M14 barrel is a pretty complicated shape, you can find the drawing online if you look hard enough.

The swell for the chamber would equate to being 1.20" diameter for 1.25" from the front of the barrel extension to a 12 degree included taper to a .745 diameter from the 0.745" diameter it tapered to 0.627 just aft of the gas port. The portion of the barrel ahead of the gas port was a cylindrical 0.595" diameter

If the barrel is 4140 or 4150 you can probably go to these dimensions without loss of strength. If you have a 416 stainless barrel I would increase the dimension by about 30% just to be safe.

You can really remove some weight on a .308 AR type rifle if you use that profile.

Since there is very little hanging on the end of the barrel (unlike the M14 or FAL) the light profile works better.

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Quoted:

Quoted:

Weren't the original M14 barrels like .650 under the hand guards??

The M14 barrel is a pretty complicated shape, you can find the drawing online if you look hard enough.

The swell for the chamber would equate to being 1.20" diameter for 1.25" from the front of the barrel extension to a 12 degree included taper to a .745 diameter from the 0.745" diameter it tapered to 0.627 just aft of the gas port. The portion of the barrel ahead of the gas port was a cylindrical 0.595" diameter

If the barrel is 4140 or 4150 you can probably go to these dimensions without loss of strength. If you have a 416 stainless barrel I would increase the dimension by about 30% just to be safe.

You can really remove some weight on a .308 AR type rifle if you use that profile.

Quoted:
Oh, and as far as accuracy goes, the thinner barrel doesn't seem to have affected things one little bit.

Since there is very little hanging on the end of the barrel (unlike the M14 or FAL) the light profile works better.

.595 is the diameter at the Flash Suppressor seal, the barrel itself is actually .585. You also have .045 grooves cut into that .585 for the last 1.5". At the muzzle, it steps down to .475! Of course, all that is on a 22" bbl, so it's a long way from peak pressure.

The M14 is .685 at 7" (just before the Op Rod Guide seat) and .670 at 8" (just after it).

Posted: 4/10/2015 9:04:44 PM EDT

[#32]

Quote History

Quoted:
right, so if a 308 bullet base is about 1.75" from the breech, per that chart i posted on pg 1, after about 6.5" of bullet travel, the beginning of my gas journal, pressures coudl still be up at maybe 32,500 psi, lets say 33k. What would be a 1.5 safety factor for that? Or for 35k psi? Can I get the formula for that?

Armalite commercial steel woudl be 4140 right? 4150 is the mil spec?

The .600 diameter of the G2 barrel starts in front of the gas journal. The gas journal is long, for the FSB, so about 2", so in reality, ther barrel doesn't hit .600 till about 9.5" from the upprer receiver I reckon. Though my gas system is longer, the front of my gas journal is in about the same place.

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Less than 0.5 inch diameter, even with a health safety factor of 2.0.

I wouldn't go that light for stiffness reasons.

41v50, 4150, 4140, they are all about the same. 128, 000 to 140.000 psi

For simplicity and stiffness sake, (and not to mention safety), I wouldn't make a barrel lighter than a straight 0.745" from the chamber swell to the gas block journal, then 0.590" to the muzzle. (With appropriate enlargements for the gas block and muzzle threads).

For the absolute minimum, talk to a barrel maker, they would know best what is reasonably safe and what they are willing to sell you.

Posted: 4/28/2015 2:50:31 AM EDT

[#33]

Okay, well, I will try like hell to get someone to do a taper from .750 to .625 behind the gas block, then to go from .600 to .500 from the gas block to the muzzle. Or abotu like that. I will have to draw it out and make sure it looks alright.

I am not worried about the stiffness so much. I figure i can put the can on it if I want to dampen harmonics.

I order a handguard and all manner of stuff for this. As soon as I can I will pull the barrel and send it off for mahcining. I'll keep you updated and post pics.

Posted: 4/28/2015 1:31:56 PM EDT

[#34]

Here is the drawing I used to make my light weight barrel, if you want it. I started with an existing barrel assembly, so some of the information isn't there. Just note, this is a rifle length AR 10 (longer gas tube) barrel. You should check the distance to the gas block (the 12.021 dimension) and the length of the gas block journal (1.125 dim) against your gas tube and gas block length.

There was an error on the originally posted drawing. The extension flange was drawn .250" wide. Corrected drawing uploaded 1/5/15

Posted: 5/1/2015 4:09:11 AM EDT

[#35]

thx for sharing your drawing, so it is .745 straight to the gas journal.

when does the barrel start at .745" from the back of hte extension or the extension flange??

Posted: 5/1/2015 4:52:40 AM EDT

[#36]

broke it down. Htere is the barrel:

Stripped, per my china freight scale, it is 880 g or 31.04 oz or 1.94 lbs. it is basically .750, maybe .740 ahead of the gas block...

Is it just me or does the chamber are look too large or something...

fuck yes it does compared to that print above...

Yeah, so its straight 1.205" OD for 2" from the front of the extension flange, then that tapered section is 1" long. So, the barrel doesn't hit .750 till 3 1/16" from the front of the flange or 4 1/2" from the back of the barrel extension...

Looks like that taper could start close to the breech, ending at the same place it does now, and w/o that stupid beveled step looking business.

Here is adco's pic of ball dimpling... It is probably a 5.56 barrel though.

I got the SLR handguard, it is about 5 oz lighter than what I had before, maybe 5.25 oz if I inlcude a short front sight rail which is integral to the new SLR handguard to make an apples to apples comparison, and the gas block is a tad lighter too. Feels better already but I have to get hte barrel a little lighter, I really want to see how far I can go here.

Posted: 5/1/2015 10:14:12 AM EDT

[#37]

Quote History

Quoted:
thx for sharing your drawing, so it is .745 straight to the gas journal.

when does the barrel start at .745" from the back of hte extension or the extension flange??

View Quote

Yes. it is a straight .745" diameter, easier to machine that way and doesn't leave that much extra weight

Those distances are governed by the taper. Since there is a +/- 2 degree tolerance on the the included angle, the distance can vary almost 3/4 inch.

Nominally, 2.141" and 9.606" from the start of the big taper.

You can, if you want, define the two end points and let the taper angle be a variable, but when it comes to machining it, the taper tolerance will have to be really tight to get the two surfaces to meet. It is easier to machine if you set the taper first, then advance along the taper until you get to the diameter you want.

EDIT: the gas block swell is .875" diam.

Posted: 5/1/2015 10:26:26 AM EDT

[#38]

Quote History

Quoted:
broke it down. Htere is the barrel:
http://i62.tinypic.com/1o8tfr.jpg

Stripped, per my china freight scale, it is 880 g or 31.04 oz or 1.94 lbs. it is basically .750, maybe .740 ahead of the gas block...

Is it just me or does the chamber are look too large or something...

fuck yes it does compared to that print above...

Yeah, so its straight 1.205" OD for 2" from the front of the extension flange, then that tapered section is 1" long. So, the barrel doesn't hit .750 till 3 1/16" from the front of the flange or 4 1/2" from the back of the barrel extension...

Looks like that taper could start close to the breech, ending at the same place it does now, and w/o that stupid beveled step looking business.

Here is adco's pic of ball dimpling... It is probably a 5.56 barrel though.
https://www.adcofirearms.com/detailpics/underhgdimple.jpg

I got the SLR handguard, it is about 5 oz lighter than what I had before, maybe 5.25 oz if I inlcude a short front sight rail which is integral to the new SLR handguard to make an apples to apples comparison, and the gas block is a tad lighter too. Feels better already but I have to get hte barrel a little lighter, I really want to see how far I can go here.

View Quote

Yes, most of the barrels you buy have about 2" of chamber swell from the front of the barrel extension flange to the start of the taper. A little generous in my opinion.

My profile is based on a Lilja's AR10704 barrel

Posted: 5/1/2015 1:32:10 PM EDT

[#39]

I really appreciate the replies. You are talking over my head a bit. I dont' know anything about prints or machining setup etc.

So you are saying that the barrel is straight for 1.24" from the front of the flange, where the taper to .745 begins? Or is that 1.24 figure an OD figure? What is that distance where the taper down begins, from the BE flange front?

I had hoped to tell the machinists:
-OD X" from A" to B" from the back, then taper from X to Y between B & C, then taper from Y to Z from C to D..." and so forth. Or, is the machinist going to need angles?

Posted: 5/1/2015 2:23:58 PM EDT

[#40]

Quote History

Quoted:
I really appreciate the replies. You are talking over my head a bit. I dont' know anything about prints or machining setup etc.

So you are saying that the barrel is straight for 1.24" from the front of the flange, where the taper to .745 begins? Or is that 1.24 figure an OD figure? What is that distance where the taper down begins, from the BE flange front?

I had hoped to tell the machinists:
-OD X" from A" to B" from the back, then taper from X to Y between B & C, then taper from Y to Z from C to D..." and so forth. Or, is the machinist going to need angles?

View Quote

The barrel extension flange is a convenient point to measure from when turning a barrel, but the barrel actually starts about 5/8 inch in front of the flange (the line ahead o flange on the drawing)

The diameters aren't noted as I assume you are starting from an existing barreled assembly with some contouring.

First cut - Starting from the flange:

1.20" diameter for 1.24" to start of first taper.

12 degree taper until the diameter is 0.745".

0.745" diameter for approximately 7.75" then start to taper out to the gas block diameter (whatever that was originally).

Then ahead of the gas block seat an appropriate amount for your particular gas block, step down to 0.595" and leave enough of a swell for the muzzle device to seat.

PM me if you what a full drawing to give to your machinist.

Posted: 5/5/2015 1:23:18 AM EDT

[#41]

Quote History

Quoted:

Yes, most of the barrels you buy have about 2" of chamber swell from the front of the barrel extension flange to the start of the taper. A little generous in my opinion.

My profile is based on a Lilja's AR10704 barrel

View Quote

Did you mean lija's 10740 barrel?
http://riflebarrels.com/shop/ar10-308/308-ar10740-drop-in/

I will show their print to ADCO so they feel confident about reshaping the chamber area. This Lija barrel is SS too, FWIW, so they shouldnt' have any qualms about doing this to my CS barrel.

I see that your measurement of 1.24" from the flange to the start of the taper down comes out the same as their 2.60" from the back of the BE.

Now for my barrel, the distance from the back of the BE to where the barrel is .750 OD is 4.48," or 3.05"...