AR-15 barrels are self headspacing

Discussion in 'General Military' started by golem1961, Jul 23, 2007.

  1. golem1961

    golem1961 G&G Newbie

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    I always hear people say to buy a complete upper to avoid headspace issues and I would like to ask what those issues could possibly be given the fact that AR-15 barrels are self headspacing. There is no adjustment that you can make that will impact headspacing in an AR-15. The only issue that can be addressed by the amount of torque one puts on a barrel/upper receiver combination is whether or not you have a good seat or if you are stripping the threads. The correct amount of torque on the barrel nut is 50 ftpounds. The only thing you have to remember besides that is when aligning the barrel nut with the gas tube opening is to tighten not loosen. I have built ARs for over twenty years and I was tutored by a retired COLT assembler and I'm sure of this information. I hope this helps.
  2. Beer Forever

    Beer Forever G&G Newbie

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    That is interesting. I haven't worried too much about head spacing because of the fact that I always get complete uppers, but only because I have found a few really killer deals.
    Last edited: Jul 24, 2007
  3. Pred

    Pred Suspended

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    It's very, VERY rare that you would get a problem with headspacing on an AR. I have no problem buying an upper(from a good company) and a bolt(from another good company), slapping it together, and taking it right to the range. The AR is a lot like the old Mosin's, in the sense that most any part from most any gun will work in any AR.
  4. lefty o

    lefty o G&G Enthusiast

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    i wouldnt go so far as to say ar barrels are self headspacing, cuz it aint so. now because of modern machining, your odds of putting one together and getting correct headspace is pretty good, but not guaranteed. unless of course the pre-chambered barrel you purchased also comes with matching bolt. without a set of guages, you can not guarantee your headspace is in spec. never assume anything.
  5. samuel

    samuel G&G Newbie

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    Correct!!!Because the housing is aluminum the bolt lugs must be locked into the steel breech.But with multiple companys making bolts and barrels they should be checked with guages. sam.
    Last edited: Jul 24, 2007
  6. golem1961

    golem1961 G&G Newbie

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    I agree that a go and or no go guage should be used when assembling any upper I am just stating that if you get a no go, there is nothing you can do about it except to swap out the barrel or check the bolt for defects. there is no adjustment that you can make to bring it into spec. the locking lugs are machined into the barrel and that determines the headspace, therefore, it matters not whether the upper was assembled by Colt, Dpms, Bushmaster, etc. or Bubba in his garage. as long as the barrel is seated headspace will not be affected. I know this is probably a hot button issue for some, however I have seen multiple posts from people perpetuating the myth that headspacing can be affected by the manner in which the upper is assembled and that just isnt true. Seat the barrel, torque the barrel nut to 50 ftpounds, and remember to tighten, not loosen to align the gastube and you will never go wrong. If for same reason you have a headspacing issue, send the barrel or bolt back for a replacement.
  7. lefty o

    lefty o G&G Enthusiast

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    i think you just need to state what your saying differently. when your dealing with a chrome lined barrel, all you can do is swap bolts and see if you get lucky. with a non lined barrel, as long as the headspace is short, you still have options. might state that chrome lined barrels have non adjustable headspace, instead of self headspacing.
  8. golem1961

    golem1961 G&G Newbie

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    all I,m saying is that you can not affect headspace by barreling an ar15 upper receiver
    Last edited: Jul 25, 2007
  9. lefty o

    lefty o G&G Enthusiast

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    but that doesnt mean that it is going to headspace correctly when you slap the parts together. thats all im saying!
  10. Big Dog

    Big Dog Dinosaur Wrangler Forum Contributor

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  11. Beer Forever

    Beer Forever G&G Newbie

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  12. lefty o

    lefty o G&G Enthusiast

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    we are getting along. not a harsh word has been said.
  13. Big Dog

    Big Dog Dinosaur Wrangler Forum Contributor

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  14. samuel

    samuel G&G Newbie

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    Headspacing is misunderstood by most.A proper headspacing will not go over .006" and the magic # is .003" The same as Ezaerln stated with the BMG.50.In auto-loaders it may be over .003" but in most cases isnt.If you go under .003" you may find you cant shoot some brands of ammo or neck resize.This goes for any type of action.They have come up with what they call "field size" guages which means you can close the action on rhe "no go" guage but not on the "field" guage.This means you can fire factory cartridges safely but probably wont be able to resize so reloads will chamber.Believe it or not,full resizing does not bring a casing back to original specs.With a "field" headspacing you can reload by using a collet neck resizing die.You just cant chamber those cartridges in any other gun.This is why LeftyO says the headspace might not be right.If you have too much headspace as long as it isnt over .006" they will work but they may be hard to resize.If you are under .003 you may and probably will have chambering problems.Some brands may chamber but others wont. sam.
  15. JoeLee

    JoeLee Suspended

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    golem1961,I had this on file I hope it helps.
    -------------------------------------------------
    The current military specifications for the M16 series call for headspace of not less than 1.4646” and not more than 1.4706” on a new rifle. The commercial Forster/SAAMI gauges are marked in 1,000ths of an inch instead the 10,000ths of an inch that the dedicated M16 gauges are. When installing a new barrel, the commercial gauge measuring 1.465" can be used in place of the M16 "Go" gauge. Consider the 4/10,000ths of an inch difference an extra margin of safety. The commercial 1.470" gauge (the commercial "Field" gauge) can be used to confirm that the headspace is within spec for a new rifle. Unfortunately, there are no gauges available to measure between 1.470" and 1.4730", the latter being the measurement on the M16 "Field" gauge.

    The M16 Field Service gauge measures a dimension of greater than 1.4730”. A rifle with a bolt that does not close on a Field Service gauge is considered safe to fire by the army, but not the Marines who use the military "No-Go" gauge for that measurement. Without using military Go or No-Go gauges, or commercial gauges marked with their measurements, you cannot know whether or not it is truly in spec. An interesting tidbit of information is that the Colt M16/AR15 Field gauge measures a dimension in excess of 1.4736”. It seems that the military has built in a bit more of a safety factor with their gauge.

    You should remove the extractor parts and the ejector, which normally requires four hands. Removing the ejector is a two-handed job if you have the proper tools, specifically a Sinclair Bolt Vice for the AR15.

    You can’t really reset the headspace on an AR15 with a chrome-lined chamber. That is set when the barrel extension is installed on the barrel. If you have an unlined barrel, with short headspace, a gunsmith can adjust it by cutting the chamber deeper with the appropriate chambering reamers. If the headspace is long, or the chamber chrome-lined, the only option is to try a different bolt until headspace checks good, or you run out of bolts. Then it is time for a new barrel.


    [​IMG]
    To recap, the specs are as follows:

    New Rifle Headspace: 1.4646" to 1.4706"
    SAMMI headspace gauges to use: 1.465" and 1.470"

    Unsafe Rifle Headspace: 1.4736"
    Use the Colt M16/AR15 Field Gauge: 1.4736" or,
    The US military gauge: 1.4730”

    Note: The Forster/SAAMI "No-Go" gauge measures 1.467"

    Sources for Gauges -
    Brownells: www.Brownells.com SAAMI gauges and a whole lot of new AR15 parts and accessories.
    Sarco: www.sarcoinc.com Military “Field” gauge as well as other maintenance items and M16/AR15 parts and accessories.
  16. samuel

    samuel G&G Newbie

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    Thanks for looking all of the specs up and the info on where to get guages.Often headspace is the cause of problems in not only AR type but any type of action.Again thanks. sam.
  17. golem1961

    golem1961 G&G Newbie

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    Okay, how about this. Anyone that can change the headspace on an AR15 by attaching it to a receiver tell me how. You can't rechamber the barrel, you can only attach it to the receiver. Can you do it?
    Last edited: Jul 28, 2007
  18. otubrab

    otubrab G&G Newbie

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    I don't suppose very many people believe the claim made above, but for those who are inclined (to believe it)... well, it really is incorrect. It is easy to run into headspacing problems on an AR 15, and there is no such thing as "self headspacing." Headspace on the AR15 in either .223 or 5.56, as with most shouldered cartridges, is the distance between the boltface plan and a datum line on the chamber shoulder (belted cartridges are the exception). On an AR15, this distance depends on 3 parts--the bolt, the barrel extension (which is really the receiver), and the barrel itself, or more specifically the chambering of the barrel. Proper headspacing depends on the proper assembly and dimensioning these parts. It's possible to grab these parts off the shelf, assemble them yourself, and have the headspace come out right. But it is by no means certain.

    What to do if headspace is short:
    Ream the chamber a little deeper. Most gun owners don't have the appropriate chamber reamer (they' expensive) so this most often is done most economically by a gunsmith. This is a go-slow operation, and I know of no way other than carefully to turn the reamer slowly and reasemlble and check frequently.


    What to do if the headspace is long:
    Much more of a nuisance. You have to set the barrel back exactly an integer number of barrel threads (to preserver the relationship of the gas hole, the extension indexing nub, and the bolt). Usually, one thread works. This, of course, requires a lathe. Once the barrel is set back, you must chamber ream to the appropriate headspace.

    Don't do this:
    Try to increase headspace by removing material from the rear of the bolt lugs or the front of the extension lugs.

    When do you need to check headspace?
    Whenever any of these three change: bolt, extension, barrel.

    If you're assembling one yourself:
    Biggest issue: getting the barrel extension in the correct relationship to the bolt and the gas port in the barrel (once the extension nub aligns with the gas port, the bolt relationship "should" be right). (This alighment doesn't guarantee the shoulder datum in the chamber will be correct for your bolt.) Adjustments require lathe work (and possible reaming). If this alignment doesn't "just happen" you're into far more expense that if you'd bought an assemble upper. In general, you can get away with it if you're not too fussy about headspace. Myself, I don't like it when headspace starts out at more than 0.001 above SAAMI minimum. If you're happy with 0.006 or 0.008 you stand a fair chance of success.

    What happens with usage?
    Headspace increases. Lug surfaces deform and wear , moving the bolt face plane back. The chamber shoulder moves forward a small amout, due to tolerance takeup, plastic deformation, and errosion. If you start +0.008 with a new rifle, you can easily exceed SAAMI specs within 2000 rounds.

    Good idea:
    Having upper built by reputiable builder, then check the headspace yourself.

    Bad idea:
    Assembling an upper without a specific plan to create proper headspace, to check it, and to correct it if necessary. If anything is wrong, the parts are not going to "self-adjust" anything.
  19. madcratebuilder

    madcratebuilder G&G Newbie

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    Good explanation otubrab, Welcome to the forum, excellent first post.
  20. otubrab

    otubrab G&G Newbie

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    Way incorrect: the locking lugs are not machined into the barrel, they're in the barrel extension (a different part from the barrel). The lugs in extension affect the headspace through their relationship to the bolt. The barrel itself affects headspace through the position of the chamber shoulder datum. This longitudinal relationship can be adjusted by moving the barrel backward relative the the threads in the barrel extension. This is done on a lathe by removing shoulder material on the outside of the barrel.
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