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Posted: 8/9/2008 9:14:56 AM EDT
| What is headspacing and do I ned to worry about it that much? I have been hearing a few things here and there that I should. Maybe someone could explain it too me as I just finished by first build and never looked into it. Thanks in advance |
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Headspace is the length of your chamber. If your chamber is too short, the cartridges won't fit into it. In practice, this is rarely a problem. If your chamber is too long, your cartridge will rattle around in there, and it may cause problems. Since this is an AR forum, I am going to assume you are shooting a bottleneck rimless case. The headspace on cartridges of this type is defined between the base and a datum on the shoulder of the case. The most common problem encountered in this situation with excessive headspace is the case will stretch to fill the void and rupture. This is crucial if you reload, as you will get ruptured cases after only a couple of loadings. The headspace is set by how deeply the extension is screwed onto the barrel. Adjustment requires removing the barrel, removing the extension from the barrel, removing metal from the extension or barrel and reinstalling. It is not a job for the home gunsmith. You can check the headspace with a gauge if you like, but excessive headspace is virtually unheard of in AR type rifles. |
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To add, if factory cartridges don't chamber or your bolt won't go into battery, then your chamber may have been cut short (insufficient HS). If you experience casehead separations, then your chamber may have been cut long (excessive HS). If you encounter neither of these conditions, I wouldn't worry about it. Mfg's are pretty good about getting everything squared away when they ream the chamber and set the barrel extension. Once those are set, there's little you can do other than swap bolts out to fine tune things, and even then you're apt to only gain or lose maybe a couple thousandths at the very most. GO gauges are set for 1.464 NO GO are set for 1.467 FIELD are set for 1.474 The goal is to have your chamber cut between the GO and NO GO dimension. But I wouldn't bother popping for gauges ($26 per gauge) unless I'm experiencing a problem which leads me to believe I'm either too short or too long. |
It is not a job for anyone. It is possible to do so, but it is not economically feasable. A barrel maker can make you a new AR-15 barrel for less time and cost. There is no adjustment for headspace. It is made to be right when the barrel is manufactured. Well, there is one adjustment. If the headspace is insufficient, the chamber can be reamed. But barrel extensions are not removed and replaced. Nor are they intended to be. If headspace is excessive, the barrel is scrap iron. Barrels and their extensions (really, a barrel assembly, and this is how they are sold to us for AR-15's) are made to a spec. Bolts are made to a spec. Even with "stacked tolerances" on the bad side, the bolt plus barrel assembly will be within tolerance, that is, good to go. Particular bolts are not specially fitted to particular barrels. A bolt should fit any barrel, and a barrel should accept any bolt. If there is a problem, either the bolt or the barrel is out of spec. Also, there is nothing you can do when installing a barrel to a receiver that affects headspace. The receiver and barrel nut are not in this dogfight. It is just the relationship of the barrel assembly and bolt. That is all that matters. |
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If you want to know the correct definition of headspace, follow this link: http://www.fulton-armory.com/headspace.htm A partial quote: In practical terms, headspace is the clearance allowed between the base of the cartridge case and the face of the bolt. The position of the cartridge in the chamber is controlled in a variety of ways depending on the type of cartridge case involved. Most rimless automatic pistol cartridges (e.g., 9x19mm Parabellum, .45 ACP) are positioned by the case mouth resting on the front edge of the chamber. Rimmed cartridges (e.g., .45 Colt, .30/30 Winchester) are positioned in the chamber by the face of the rim resting on the rim recess at the back of the chamber. Belted magnum cases (e.g., .300 Win Mag., .458 Win Mag.) are positioned by the belt resting on the recess provided for it at the rear of the chamber, much as rimmed cartridges are. Finally, bottle necked rimless cartridges are positioned by the shoulder of the case resting against the shoulder of the chamber. |
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As eweloader quoted, the 5.56mm/.223 case "headspaces" on its shoulder. That means that the precise relationship between the case head and shoulder is important to the case fitting and performing in the chamber. The "headspace" measurement for this cartridge is based on something called a "datum line" which is at a specific point on the shoulder, but if the shoulder angle is right (it's hard not to be), then you can just consider the whole shoulder to be the headspace point-at least for practical purposes. Now the good news for an AR builder: headspace for an AR is determined by how the bolt is machined and how the barrel extension is fitted to the barrel. Since both of these values are very much in the control of the manufacturer, and both are held to very tight tolerances, it's not very easy to get it wrong. And if it is wrong, there's NOTHING you can do about it at home. So what's "wrong" here? If a standard headspace gauge doesn't fit, or more precisely if the rifle won't close on a "GO" gauge or will close on a "NO GO" gauge, then there's something wrong. These gauges aren't trivially cheap, but they're not that expensive either. When I built my first AR, I actually took the rifle to a well reputed gunsmith (not a "mechanic," but someone who was a real 'smith) and asked him to check the headspace for me. He was amused and did it for free, then explained why it's not a big deal-basically what i said in the first and second sentences of this paragraph. If you worry, find someone with gauges and get it checked. After you find out that it's fine, you can relax. Because I can almost guarantee that it WILL be fine. That's a lot of words. It comes down to: if you used a barrel and bolt from some trustable source, you should be fine. For a full explanation of headspace and what it does, check out the links above and look at some reloading information. And relax! 
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