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Zero headspace is perfect


Alan Waters
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Found this question in a thread that had been closed.  [seen plenty of videos and write up about headspacing, but no one ever talks about how to correct it! So you determine that your headspacing is wrong. Then what????? Please dont say take it to a gunsmith, the point of this is DIY.] Well, unless you are a gunsmith or a machinist you have to take to one.  So, what is excessive headspace and how is it corrected? Headspace is the distance from the case head [where the primer is] to the face of the bolt when there is a cartridge in the chamber and the bolt is closed. The cartridge shoulder is being pressed firmly against the chamber shoulder. This stops the forward movement of the cartridge and the bolt applies pressure to the case head to hold the cartridge stationary when the gun fires. This distance should be Zero but usually is not. Why should this distance be Zero? Several reasons. Number one is safety. When the gun fires, if the brass cartridge case is not held captive by Zero headspace the case move to the rear as the bullet moves forward. This means the case backs out of the chamber until it contacts the boltface. It this distance is great enough, the cartridge case, unsupported by the chamber will blow a hole in the unsupported brass. Not good when 30 to 50,000 psi is that close to your face looking for a way out of the firearms action. Number two is also safety. If the case did not blow out it definitely stretched the brass, and if it is reloaded that brass now has a thin spot and may or may not fit in your chamber.The SAMMI spec. for .308 Winchester is two thousands max between go and no go. The field specs allow eight thousands. If you want to know  what the headspace is, and you should, all you need is a go gauge of the proper caliber and a set of feeler gauges from the auto parts store. The flat leaf type used to adjust valve clearance.  First, remove the extractor and the ejector. If you don't do this you are wasting your time. Cut a piece of the .001 feeler gauge small enough to fit in the bolt, up against the bolt face. Insert your go gauge and close the bolt. You will have to hold the rifle slightly barrel high to keep the feeler gauge in place. If the barrel is too high the gauge will try to slide back out so its a balancing act. If all fails place a TINY drop of oil on the bolt face under the feeler gauge.This acts as an adhesive to hold the feeler gauge in place. Understanding if you use the oil it may add a thousands or two onto the measurement. If the bolt will close on the one thousandths cut a piece of two thousandths and repeat. Keep going up in size until the bolt will not close. OK. Now suppose there is nine thousandths excessive headspace. How do you get rid of it. There is only one way. The barrel must be removed and place in a lathe. Nine thousandths material must be removed from the shoulder of the barrel where it fits against the barrel extension. Yes it could be removed from the barrel extension but the heat treat on those parts is very hard. In any event, the barrel chamber needs to move toward the boltface nine thousandths to achieve Zero headspace. Having done that, depending on how much clearance the front of the bolt locking LUGS had from the rear of the barrel,  before the barrel was set back, it my be necessary to remove another nine thousandths from the rear of the barrel to allow clearance for the front of the bolt locking LUGS.  If, by mistake, too much was removed from the barrel shoulder and the barrel goes too far to the rear, the only fix is to use a chamber reamer of the correct caliber to deepen the chamber to the point where the go gauge will fit properly. 

I have tried to write this so that the person with little or no knowledge of the subject may be able to understand it. That was me 40 years ago and all of you at some point.

Note. Head space measurement on a blueprint will be read from a point on the shoulder of the chamber as can be seen in the drawing below. 

http://forums.thecmp.org/showthread.php?t=146769                       Regards to all, Alan Waters

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46 minutes ago, jtallen83 said:

Thank you for a well written explanation, now I think I understand the purpose of the "match rifle" headspace gauge sets like this, am I on track?

https://www.brownells.com/gunsmith-tools-supplies/measuring-tools/headspace-gauges/223-5-56mm-match-rifle-headspace-gauge-set-prod42236.aspx

I was not familiar with that set but it appears they are doing the same thing that can be done with the feeler gauges.

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1 hour ago, jtallen83 said:

Thank you for a well written explanation, now I think I understand the purpose of the "match rifle" headspace gauge sets like this, am I on track?

https://www.brownells.com/gunsmith-tools-supplies/measuring-tools/headspace-gauges/223-5-56mm-match-rifle-headspace-gauge-set-prod42236.aspx

 

4 minutes ago, jtallen83 said:

And your method is a good bit cheaper! :thumbup:

I got a complete set of those from Midway for my build.  Feeler gauges would have worked as well, but these just look so cool sitting on the shelf in your gun room ?

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Only problem with the method you describe is, (and I assume you are describing correcting excessive head space on an AR platform), if you remove even .005" from the shoulder of the barrel you will now have a gas port that is no longer at 12 o’clock, the thread on the DPMS style large frame extensions is 16 TPI, so for every 360 deg the thread advances .0625", .005" will rotate the gas port 2 deg, that's 2 deg is at the bore axis, the further from the axis you go, ie, to the outside diameter of the barrel the larger the 2 deg steps become, it will rapidly advance the gas port around the OD of the barrel the larger the barrel diameter.

I regularly set barrels back on bolt rifles, particularly a lot of F class rifles as the owners are exceptionally pedantic about throat erosion and the process is what you describe but unfortunately it doesn't work with the AR platform.

 

Edited by 308kiwi
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12 minutes ago, 308kiwi said:

Only problem with the method you describe is, (and I assume you are describing correcting excessive head space on an AR platform), if you remove even .005" from the shoulder of the barrel you will now have a gas port that is no longer at 12 o’clock, the thread on the DPMS style large frame extensions is 16 TPI, so for every 360 deg the thread advances .0625", .005" will rotate the gas port 1 deg, that 1 deg is at the bore axis, the further from the axis you go, ie, to the outside diameter of the barrel the larger the 1 deg steps become, it will rapidly advance the gas port around the OD of the barrel the larger the barrel diameter.

I regularly set barrels back on bolt rifles, particularly a lot of F class rifles as the owners are exceptionally pedantic about throat erosion and the process is what you describe but unfortunately it doesn't work with the AR platform.

 

Have to disagree on the barrel advancement. How could the gas port advance more on a large diameter barrel? If you set the barrel back enough to index the gas port 90 degrees it would not matter if the barrel is .750 or two inches in diameter. The distance traveled would be greater for the two inch barrel but it would still be 90 degrees.

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8 minutes ago, Alan Waters said:

It would work if the set back was done on the barrel extension flange would it not? 

Unfortunately not, it's the one drawback of gas operated rifles, the gas port will always move around the barrel diameter regardless of where you remove material to set the barrel back to correct excess head space.

I've spent many hours trying to figure a way to do this and haven't yet come up with one that will work, except you could remove exactly .0625" from the shoulder and the breech face, this will set the chamber back .0625 and realign the gas port at 12 o'clock and then you would re-cut the chamber with a reamer to the correct head space. But to try to do this to only set the chamber/head space back a few thou doesn't work.

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1 minute ago, Alan Waters said:

Have to disagree on the barrel advancement. How could the gas port advance more on a large diameter barrel? If you set the barrel back enough to index the gas port 90 degrees it would not matter if the barrel is .750 or two inches in diameter. The distance traveled would be greater for the two inch barrel but it would still be 90 degrees.

That's not what I said, yes it would still only advance the gas port 90 degrees, BUT, the distance between each degree would be greater the further out from the axis you go, liken it to 1 MOA, 1MOA equals 1" @ 100 yards, but at 200 yards that 1" is now 2" and so on.

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Just now, Alan Waters said:

That is incorrect. Setting back barrel extension does not turn the barrel. The barrel is pinned on top so the only direction it can move it to the rear. You may have to deepen the pin slot.

Sorry I don't get your point, if you set the barrel extension back?, back from where?, you are wanting to move the chamber back to reduce excess head space, the barrel extension is simply going to follow.

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8 minutes ago, 308kiwi said:

Sorry I don't get your point, if you set the barrel extension back?, back from where?, you are wanting to move the chamber back to reduce excess head space, the barrel extension is simply going to follow.

 

Agreeing that the barrel and extension become one after proper assembly, if material is removed from the rear of the barrel extension flange, the chamber will move rearward an equal amount. You could get the same effect by setting back the receiver face.

Edited by Alan Waters
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Alan, far be it for me to tell you anything about machining anything - but if you trim that barrel extension to lessen what you're trying to - you're working with a threaded barrel extension, that has to be timed.  As soon as you shorten that barrel extension to achieve what you're trying to - your bolt lugs will slam into that untimed extension, and it will never lock.

Maybe I don't even understand what you're trying to do here?...

One word of advice, though - gas guns are WAY different than working on bolt guns, so alot of the bolt gun experience you have will be useful.  Alot of the bolt gun experience you have won't amount to a hill of beans on the gas guns.

The barrel extension is the first operation, in building these barrels.  You don't mess with it after that. You can't mess with it after that. When it's done - it's really done. 

Edited by 98Z5V
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11 minutes ago, Alan Waters said:

 

Agreeing that the barrel and extension become one after proper assembly, if material is removed from the rear of the barrel extension flange, the chamber will move rearward an equal amount. You could get the same effect by setting back the receiver face.

All that is achieving is setting the entire barrel/barrel extension rearward, it doesn't change the relationship between the barrel extension locking lugs and the chamber, which is where you are measuring head space between.

The only way to set a barrel back to remove excess head space in an AR system is to move the barrel shoulder forward by the exact amount of the thread advance, (.0625" in the case of a 308 AR), then remove the same amount from the breech face, (end of the barrel trunion), to set your bolt lug clearance then re-cut the chamber with a chamber reamer to the correct head space.

And even this process is fraught with potential disaster as you would then have to torque the extension back to the barrel to the same point that it was originally installed or you would STILL end up with a gas port that wasn't at 12 o'clock.  I torque extensions on both small and large frame AR's to 125 lb/ft, to achieve perfect alignment you would be tightening the extension by degrees rather than a pre-set torque limit.

 

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1 minute ago, 308kiwi said:

And even this process is fraught with potential disaster as you would then have to torque the extension back to the barrel to the same point that it was originally installed or you would STILL end up with a gas port that wasn't at 12 o'clock.  I torque extensions on both small and large frame AR's to 125 lb/ft, to achieve perfect alignment you would be tightening the extension by degrees rather than a pre-set torque limit.

 

And that torque should be 175 lb/ft.  Good luck finding that spec in a military manual.  It took me years to find that, and searching these days doesn't turn it up. 

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1 minute ago, 98Z5V said:

And that torque should be 175 lb/ft.  Good luck finding that spec in a military manual.  It took me years to find that, and searching these days doesn't turn it up. 

I searched for some time to find that spec to no avail, so went with 125 lb/ft and, (much to my objection because I hate the poop), high strength Loctite.

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5 minutes ago, 308kiwi said:

All that is achieving is setting the entire barrel/barrel extension rearward

Well, forward in the sense of heading towards the muzzle and potentially leaving the BCG riding farther forward. The shoulder on the barrel extension is theoretically still resting on the receiver face. Otherwise, completely agree with your assessment. Simplest solution seems to be to wait until correct headspace is established and then drill the gas port. ?

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4 minutes ago, 308kiwi said:

All that is achieving is setting the entire barrel/barrel extension rearward, it doesn't change the relationship between the barrel extension locking lugs and the chamber, which is where you are measuring head space between.

The only way to set a barrel back to remove excess head space in an AR system is to move the barrel shoulder forward by the exact amount of the thread advance, (.0625" in the case of a 308 AR), then remove the same amount from the breech face, (end of the barrel trunion), to set your bolt lug clearance then re-cut the chamber with a chamber reamer to the correct head space.

And even this process is fraught with potential disaster as you would then have to torque the extension back to the barrel to the same point that it was originally installed or you would STILL end up with a gas port that wasn't at 12 o'clock.  I torque extensions on both small and large frame AR's to 125 lb/ft, to achieve perfect alignment you would be tightening the extension by degrees rather than a pre-set torque limit.

 

I'm not measuring the headspace between the the barrel extension and the chamber. That is not headspace. If you set the barrel/extension back it locates the chamber closer to the bolt face.

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Just now, 308kiwi said:

I searched for some time to find that spec to no avail, so went with 125 lb/ft and, (much to my objection because I hate the poop), high strength Loctite.

Use Rock-Sett, brother.  Never worry about it again.  This is the manufacturer - Flexbar:

http://www.flexbar.com/shop/pc/ROCKSETT-2-OZ-p4019.htm

I found that spec, years ago, in a military manual reference.  The 'net is so polluted with information now, that finding it is a fruitless waste of time.  I will find it again, someday, and save the shiit out of it when I do.  And post it here with an immediate request that it be pinned...  :thumbup:

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1 minute ago, Alan Waters said:

I'm not measuring the headspace between the the barrel extension and the chamber. That is not headspace. If you set the barrel/extension back it locates the chamber closer to the bolt face.

How are you going to accomplish that on an AR platform, and keep the BCG and recoil system in check?...

You set that back, and you set the BCG back.  As soon as you do that, you set the cam pin back - and it MUST rotate in the upper receiver cam pin cutout...

There's alot more here than there is in a bolt gun...

Edited by 98Z5V
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1 minute ago, Matt.Cross said:

Well, forward in the sense of heading towards the muzzle and potentially leaving the BCG riding farther forward. The shoulder on the barrel extension is theoretically still resting on the receiver face. Otherwise, completely agree with your assessment. Simplest solution seems to be to wait until correct headspace is established and then drill the gas port. ?

Alan suggested removing material from the rear of the barrel extension shoulder or the front of the receiver face, either would shift the entire assembly rearward.

But yes absolutely, Fit the barrel extension, cut the chamber to head space THEN drill the gas port?

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2 minutes ago, 98Z5V said:

Use Rock-Sett, brother.  Never worry about it again.  This is the manufacturer - Flexbar:

http://www.flexbar.com/shop/pc/ROCKSETT-2-OZ-p4019.htm

I found that spec, years ago, in a military manual reference.  The 'net is so polluted with information now, that finding it is a fruitless waste of time.  I will find it again, someday, and save the shiit out of it when I do.  And post it here with an immediate request that it be pinned...  :thumbup:

Thanks 98:thumbup:

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