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Posted: 4/20/2003 11:28:24 AM EDT
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OK I have little question regarding headspacing. I am familiar with FALs and AKs for headspacing, but not ARs If one buys a barrel or upper with the barrel extension already installed, then how does one set the headspace with the bolt? I know you check it with a go/no-go gauges, but if you buy the bolt separately, then what do you do - go through a pile of bolts until you get one that headspaces? I would think that one would need to buy the upper with a bolt already headspaced for the weapon from the mfg. Tom |
| I've bought new and used parts to build AR's and have never had a headspace problem. I bought a GI gauge because it was easier to use than the 3 pack GO/NO-GO/FIELD gauges. Although I've never had a problem, I think if my bolt closed on the gauge I'd get new parts. |
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I have the a similar question and a search to the archives can't find where it is covered. I've got barrels threaded for the barrel extension, the extension, the extension pin, and the socket tool for the extension. Do I lubricate the barrel threads before I start? What tricks make starting the threading easier? When do I stop screwing the extension home? Any tricks to installing the pin or just knock it in? Thanks for your help. |
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Are your barrels already drilled for the index pin, gas port and front sight base taper pins? If so, you could have a hell of a job ahead of you. Yes, use moly anti-sieze compound on the extension threads. If your barrel's a virgin, and has only been threaded for the barrel extension, then you will indeed have to set headspace (presuming the chamber's already cut). Screw the chamber extension almost all the way down. Put a no-go gauge and the bolt in the chamber and rotate the bolt to engage the lugs. Continute tightening the extension until the bolt, with no-go gauge, is tight against the lugs with minimal pressure. Make a mark where the index pin hole is here. Repeat the tightening with your go gauge and Make another mark. The area between the two marks will provide your safe headspace range, and you get to decide how loose/tight you want your headspace to be within that range. I'd personally go halfway between them. Set the index pin hole of the barrel extension somewhere between your two marks, and drill for the index pin - top dead center of the installed barrel has now been set, and headspace is correct. Now you get to drill the gas port at the correct location and TDC, install your barrel nut handguard end cap and front sight base. Align the sight base to TDC and drill for the FSB taper pins. If your barrel's one of those takeoffs, already drilled, just install the extension until it's lined up for the existing index pin hole. Check your headspace. If it's good, you're good to go. If it's not, you're screwed, unless you feel like welding up the index pin hole, gas port and taper pin grooves, and then proceeding as outlined above to set proper headspace and redrill everything as if for a new undrilled barrel. Good luck. |
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Very few builders will ever build an AR having to add the barrel extension to a naked barrel. We usually buy "barrel assemblies". If you are doing as in the post above, yes, the procedure Circuits outlines is correct, and this may be done for an all out target grade AR type rifle. Here on AR15.com advice persists about "having a competent gunsmith set the headspace on your rifle". When people say this, it makes me doubt their understanding of the workings of the AR15/M16. No one but no one "sets the headspace" on an AR15 type rifle, they CHECK it. Hopefully I am wrong, and what they really mean is, "You should have a competent gunsmith CHECK your headspace." That statement I will go along with, particularly with well-used firearms/parts. If any of you have had your AR15's "headspace set" or adjusted, I ask, did you actually see this being done? Was this with new parts? Did the gunsmith charge you very much for this job? I would really like to see a video tape of YOUR gunsmith "setting" YOUR AR15's headspace. The machining, and time/labor costs involved in doing so are way out of proportion to the cost of the parts involved. Unless a highly tuned racegun is being assembled, it is easier, faster, and cheaper to simply replace the out-of-spec parts. Setting headspace is something done by a custom riflemaker (who is somewhat more than just a gunsmith) when mounting a new barrel on the receiver of a bolt action rifle. A custom-built rifle of this sort will end up costing many thousands of dollars. There is a lot of difference in construcion between a custom-built target or African big game rifle, and the AR15/M16 rifle. With a bolt action, the barrel screws directly into the front of the receiver. The bolt locks up into the receiver, not the barrel. The AR15's barrel has a barrel extension that the bolt locks into. The bolt locks into this extension, NOT the receiver. Further, it must be indexed correctly, as does the front sight assembly. It is this design that allows the upper receiver to be made of aluminum, or even hi-tech plastics, and operate safely. This is also why torque used to tighten the barrel nut has no effect on headspace. (Aside-With the AR15/M16, the bolt carrier does not enter into the headspace equation--only the bolt, barrel and its chamber, and barrel extension determine headspace. Additionally, for our purposes, the barrel and barrel extension are considered one part.) With a bolt action, the barrel may be removed, a small amount machined off where the front of the receiver contacts the barrel. Then the barrel can be remounted, and the chamber reamed to give the correct headspace. This is called "setting back" the barrel. (At the same time, an older barrel would be recrowned for a complete job.) More often, a new barrel is mounted, and this new barrel finish-reamed to desired headspace. The design and type of assembly of the AR15 precludes this. No one at GM "blueprints the engine" or any of the other engine building techniques used by race car mechanics. The engines are made to work and produce the desired power and performance by simply being bolted together. Same with the AR15/M16 design. This interchangability of parts is a necessity in military rifles. Military armorers do not have the time to spend hours custom fitting parts. Also, with the AR15/M16 rifle, by the time it is worn enough to have excessive headspace, the barrel is most likely worn enough to require replacement of the entire barrel assembly and bolt. In fact, it is probably time for a whole new upper half. Interestingly, one of the things they do in testing new pistols and rifles being considered for military use is to take apart the entire batch of test weapons and reassemble them, picking parts at random. This is to test that none of the guns were specially hand fitted or selected, and that normal production tolerences will result in a properly performing firearm. The military firearm MUST function correctly without special fitting. The resulting firearms must then pass all of the required tests, functioning safely and reliably, and do so for many rounds fired. With the AR15/M16, if the barrel and bolt are in good condition, the head space will be OK. And with new barrels and bolts from reputable suppliers (Bushmaster, Colt, DPMS, J&T, Model 1 Sales, Armalite, Olympic, Rock River, and others), you may assemble an AR15 upper and not have headspace problems. You may also buy a barrel from one supplier, and the bolt from another, again, with no problems. I also ask this, has anyone here assembled a new barrel and new bolt, and had improper headspace? I certainly have seen no reports of this here, or on any other gun website I visit. All the AR15 kabooms I have read of trace to something other than excessive headspace... high primers on reloaded ammo, or some other problem. (I WOULD AVOID cheap parts kits made from used parts, which were disassembled from worn out weapons being discarded. If some military, ours or another, is throwing this stuff away, I don't want it either.) With all the talk of checking for excessive headspace, many also advocate the "5.56 NATO" chamber over the ".223 Remington SAAMI" chamber. If the 5.56 chamber is measured with .223 gauge, it will measure on the verge of "worn out". This is discussed in the tacked threads at the top of the Build It Yourself section. It is perhaps more important to occasionally check your brass for signs of stretching. This shows up as shiny rings around the circumference, just above (about 1/4"-3/8") the case head. These shiny rings indicate your rifle is very near case head separation. |
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Great info... thanks! I am also curious if you can advise if a person has a wildcat... say a 300/221 fireball (AKA 300 Whisper (R))... it is my understanding that a person can use a 221 Fireball Head Spaceing gauge to confirm the headspacing for the larger wildcat... is this correct? So, in this context, when a person headspaces a rifle, they are really checking the distance from the case shoulder area of the chamber to the rear of the chamber where the bolt closes. Is this correct? |
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Hi Circuits, The method you describe does not mention the 150ft/lbs of torque required to tighten the barrel extension. I thought the procedure was to tighten and release the extension 3 or 4 times to bed the extension onto the shoulder of the barrel,then tighten to the required torque(150ft/lbs),drill the .125" hole for the index pin and the appropriate size hole for the gas port and then headspace using a finish chamber reamer and GO guage. Doesn't a 'loose' barrel extension present problems with accuracy etc.? |
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Quarterbore: Yes, you are correct. I am not sure about using the .221 Fireball gauges, would have to look at drawings of each chamber, to see. Sometimes with wildcats, the shoulder is also altered. There is an interesting Armalite Tech Note on headspace, specifically as it relates to military type semi-autos. Another interesting .223-based wildcat, and easily loaded by the home reloader, is the 6mm-.223, aka 6mmx45. This is a .223 necked up to 6mm, and same OAL as the .223 Rem. Thus, it fits in M16 magazines. Olympic Arms offers this in an upper assembly. Intestesting thing is it is only 150-200 fps slower than the .243 Winchester, and is legal for deer hunting in states that forbid use of .22 calibers. The drawback would be mixing up your ammo... imagine picking up a mag full of 6mm-.223 and trying to fire in your .223/5.56 upper! Fortunately, it would not chamber, though the reverse could happen, .223 Rem in the 6mm-.223 upper. |
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FAHR: Yes, one would do that if starting with an unchambered barrel blank - I didn't give the full, exhaustive procedure, since I don't like typing that much. I presumed the chamber had already been cut, and especially in the case of a chrome lined takeoff barrel, one doesn't want to go trying to ream through the chrome lined chamber to get things right. romancenturion - he's talking about barrel EXTENSION torque, not barrel nut torque to the upper receiver. |
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Fahr 451 is absolutely correct. Sorry Circuits, you are dead wrong. You MUST tighten the extension fully against the shoulder of the barrel at 150 Lb/Ft of torque. The chamber final depth is cut/finish reamed after the assembly of the extension to the barrel. You CANNOT rely on the index pin to locate (i.e. headspace) the barrel extension on the barrel. Doing so will be an invitation to disaster. Doing as he (Circuits) describes will result in the extension working (very) loose on the barrel after just a few rounds. The index pin is very soft and is only intended to locate the assembly in the upper receiver, not to locate or "lock" the extension to the barrel. Unless you REALLY know what you are doing and properly equipped, it's best not to attempt to mess with installing an extension on a barrel blank. Cutting chambers, finish reaming chambers, drilling gas ports, drilling and reaming taper pin holes is best left to the experts. Romancenturian1: I think you misunderstand what's being discussed here. You do not tighten the BARREL NUT to 150 Lb/Ft. We are discussing the BARREL EXTENSION here. That's the part that has the bolt lugs on it, not which holds the barrel assembly onto the upper receiver. As someone mentioned, your money is better spent buying a barrel assy that's ready to just screw onto the receiver. |
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Wow -- you guys are great. The barrels are take offs in good shape without throat erosion and with bright bores. The uppers are flat tops so I am installing the gas blocks without the site. The barrel extensions, Sarco, came with (surprise) the chamber end of the barrel attached and it was a pain to remove the extensions. AFM-the attached barrel pieces were very obviously shot out and I'm now wondering if I shouldn't toss these extensions and get new ones.-- yes. Only a foolish few would try to add an extension on to a barrel. All the others I've put together have come as one-piece. circuits -- I can close the existing gas hole and redrill if needed. I do not have a reamer plus the chrome chambers would make that a real pain so I'll screw the new extensions down all the way, put the pin at the top, and check the headspace. Burning paper -- thanks. I can see how for the factory assembly that would be the way to go. Russ 4777 -- I can see how the extension could back off and create excessive headspace. Many argue my money should be spent in better directions. I only do this to keep me off the streets at night. |
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Stirling 18 asked "A_Free_Man -- so what your saying is if you purchase a barrel and bolt/bolt carrier that are both milspec from a good source, you should have no issues with headspace. That would be a true statement right? Sorry, I'm working on my first." That is EXACTLY what I am saying. And I would not hesitate to buy them from two different good souces, for example, as a barrel from DPMS and a bolt from Armalite. WinnR asked "AFM-the attached barrel pieces were very obviously shot out and I'm now wondering if I shouldn't toss these extensions and get new ones.-- yes. Only a foolish few would try to add an extension on to a barrel. All the others I've put together have come as one-piece." You can buy a whole barrel assembly, new barrel, with extension already on it, barrel nut and delta ring assembly, front sight, the whole shebang, ready to mount and shoot, from J&T or Model 1 Sales for $135. These are E. R. Shaw barrels, and are pretty good general shooting barrels. At that price, why go thru any aggravation with worn out parts? |
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The whole thing about preoccupation with headspacing on military rifles comes from the days when gazillions of old worn out miltary rifles were flooding the civilian market. So, gunsmiths were manic about headspace on these old guns. They were often used as a basis for a custom rifle, a new barrel on a Mauser action, etc. But this is not the case with assembling an AR15 from good parts. IF your AR15 is well worn, or you are building a kit made from used parts, yes, I would recommend careful checking of headspace. Also, regular .223 gauges cannot be used, the semi auto chamber is looser. Go to Armalite's site and download the Tech Note on headspace. |
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