IMO, there is a critical measurement more important to AK safety than headspace, and as far as I can tell, it is getting ignored in favor of headspace. It is what I call bolt gap. Below is a picture of it being measured.
But first, here are some pictures that I think show why it is important. As you can see, the ass end of the case sticks out about the same amount as the case head is thick. Let that case stick out too far and the case wall starts becoming unsupported by the barrel. It should be obvious why that is bad. What may not be obvious is why a headspace gauge alone is not adequate. Here are some examples of what could go wrong with using a headspace guage.
1. A rifle with a short, burred or dirty chamber c hamber could check OK with headspace gauges but have too much bolt gap and a nd case exposure.
2. Even worse, I can imagine that after building such a rifle, a well-intentioned builder might decide he needs to reposition re position his barrel forward and re-pin it. This would satisfy satisfy his his headspa headspace ce gauges gauges,, but but give the rifle rifle danger dangerous ous case exposure.
3. A rifle with a sloppy or long chamber might swallow a No/Go guage, prompting the builder to push the barrel in further and re-pin, only to find that the bolt will not close reliably (or at all) because it hits on the barrel. As far as I can tell, this would only be dangerous if someone were shooting back.
How to measure bolt gap
Place a spring in the chamber to keep the bolt fully back. I use a 3/8 x 2.5 inch spring. Insert and rotate the bolt closed as far closed, and only as far closed, as the it rotates when being held by the carrier. This is not quite all the way. Use a feeler gauge to measure the gap on the left side (away from the extractor). The type of feeler with a tapered tip works best, but a regular one (like in the picture) can be used if you are careful.
I have not found a factory spec (anyone have an AK repair manual?) but I find that 6 thousands, give or take a few, is typical. If you set up for the measurement differently than I do (there are probably other acceptable ways) you will probably get different results. The main thing is to be consistent, and if you can, compare your build measurements to a factory gun. I use a Saiga and a SLR95 as reference standards.
OBTW. I have similar pictures of a brass Winchester case, if anyone is interested.
--------------------------------------Since the chamber depth is fixed regardless of where the barrel is pinned to the trunion, how is this possible? I don't see how any adjustments can be made on the rifle to change how much of the case sticks out of the chamber. It is solely a function of how the chamber was reamed. Moving the barrel and bolt in relation to each other would not change the length of unsupported case.
Am I missing something? --------------------Hey guys. You are all correct, of course. And thanks for taking an interest. I was starting to think no one would even say boo.
It seems that I didnt sell/communicate my point very well. Unfortunately, It is late right now and I need to go to bed. If you will humor me, I will try to think of a way to explain what is missing. Right now, it is not coming to me.
But fiirst, I feel like I need to say that this is not just some goofy idea that I
came up with. It a part of the standard procedure for setting up a barrel on any action that I know of. Bolt, lever, semiauto, you name it. Also, it is no accident that the AK barrel and bolt consistently come from the factory with about 7 mils gap. Finally, I think it should be fairly easy to see that you wouldnt want to have , say, 30 mils gap.
In the mean time, give it a chance if you would. Konking out here. Ozzy ----------------------------------So basically, the bolt gap would leave exposed parts of the catridge case, possibly leading to a detonation out of battery even though the bolt closes? If my understanding is correct and say a kit left romania, yugoslavia, etc with excessive bolt gap, that kit would have to either be re-reamed (ruining the bore) or tossed? ---------------------------------- Yes Blood, that is a good analogy. Firing a rifle with too much bolt gap is the equivalent having a cartridge go off out of battery. The fix would depend on the cause, which could be any one of the following: incorrectly chambered barrel, incorrectly pinned barrel, incorrectly recessed bolt head, stretched trunnion, or locking lug damage/wear. ------------------------------Sherm, If I understood your question correctly, you struck at the heart of the issue. If a re-builder is willing to assume that the barrel was chambered correctly and the bolt head is recessed correctly, then Go & No Go gauges alone are adequate. Similarly, if he is willing to assume that the bolt gap is correct and the bolt head recess is correct, then Go & No Go gauges are adequate. And so forth. In essence, there are too many variables to be checked with one type of gauge. ----------------------------Well fellas, I had hoped to create some healthy dialog on the subject, but failing that, I am going to light off on a rant. There?s a chance I will insult somebody, step on some toes, and say something that is easily misunderstood. But I am going to take the chance. I may PO someone, but at least you will know where I stand. I mean all this in the best of all possible ways.
I started this thread is because I am seeing a lot of misunderstanding about headspace in general and headspace gauges in particular. I see board experts that didn?t know the difference between headspace and outerspace until a month ago. And others who are experts because they have figured out ingenious ways to bend a piece of sheet metal and rivet it between to blocks of steel, but they have never rechambered or reset a barrel. I see guys that have read that excess headspace is dangerous and figure that the opposite must be
true. They figure that they are safe as long as they check their headspace with a gauge. And I see guys posting (here and there) that just they used their shiny new headspace gauges and found that their headspace is off a few thousands this way or that. They ask what to do and inevitably someone volunteers that he should reposition the barrel, redrill the pin hole and replace the pin with an over sized pin. The guy with the new gauges thinks this sounds good and does it without realizing that a headspace gauge does not tell the whole story. And I see old timers who have made custom rifles and handoaded wildcat cartridges for decades being blown off by the new experts. I suppose this is all just human nature, but I would sure like to straighten out at least a little of it, if I can.
So far, all of the problems that I see are based on assumptions that I think most folks don?t know they are making. I came to this realization after being scorned (here and even worse on another board) for saying that excess head space is not dangerous if you are not reloading cases. What I said is true, but it ASSUMES that the bolt gap is OK. I assumed everyone (or at least someone) was as familiar with setting up/chambering new barrels as I am. Well, you know what they say about assumptions. Anyway.
First point: Using a factory loaded round with a shim on the head is the old standard way of checking headspace, and it is actually safer than using a gauge. The reason is this: A gauge checks the distance from the bolt face to a reference point (ring) on the shoulder. It checks that this distance is a standard length. But it does not verify that this distance matches your loaded ammunition. If you use a gauge, you are ASSUMING that the manufacturer that loaded the ammunition and the manufacturer that made the gauge used the exact same dimensions, including the exact same location for the reference point on the shoulder. Considering that we are talking ammunition and arms made in various countries over a decades of time, this is not always a good assumption. On the other hand, if you check your rifle against the ammo you are going to use, you have eliminated the assumption.
Second. As I tried to describe in my first post, there are more variables involved that just the distance between the shoulder and the bolt face. For that matter, the term headspace, as commonly applied to a rimless cartridge is a misnomer. The term headspace made sense in the old rimmed cartridge days. In those days, headspace meant space for the head. But for rimless cartridges, it means space for the case. On a rimless case, headspace doesn?t tell you how much head is sticking out of the chamber. All it tells you is the distance between the shoulder and the bolt face. That dimension is nice to know, but by itself it doesn?t tell you whether a rifle is safe or not. Don?t assume that the headspace gauge checks all critical dimensions.
Third: It isn?t good enough to just check the headspace before you shoot the
gun and then forget it. You need to check it again after firing. How do you know that the barrel didn?t shift forward. There is no way that you got that barrel back in exactly the original spot. The pin may or may not have realigned things. It may have shaved. I have seen it happen and there have been several posts where guys pushed the barrel in a little farther to tighten up the headspace and then managed to get the pin back in and improve their headspace measurement. But they need to know that the barrel will shift forward when the gun is fired, if there is slack in the pin. If you can pound a barrel in and out with a hammer, you can be sure that the pressure of a cartridge can move it. I even know of one guy who sheared a makeshift pin and shot the barrel out of the trunnion and onto the ground. Don?t assume that barrel wont move a little the first time you fire it.
Forth: When you pull that barrel, you are entering the gunsmiths realm and you need to understand what is involved in correctly setting up a barrel. Because, when you check the headspace and find out it is off, you need to know how to figure out the cause. And that is why I started the post to begin with.
I could probably rant on, but that?s enough for now.
So, whose shxt list am I on? ------------------------Preacher, You would make a good straight man. And congratulations on your becoming official. I am sure you know that there is a current shortage of good preachers (although personally, I prefer the term minister to preacher). I would be happy to expand on #4. But first, I want to talk a little about bolt head recess, because it is the only one of the 3 variables that hasn?t been examined.
Rodm1, Yes I agree. I am trying to get my hands on an AK assembly or repair manual. For now, I can only provide the range of measurements that I see in my Aks. I would think you are safe as long as you stay in that range. I really don?t want to hazard a guess as to how far outside the norm anyone can go. Safety is such a subjective thing. One person might be comfortable with a one in a thousand chance of KABOOM and another might be terrified with a one in a billion chance. So the best I can do is explain how things work and leave it up to each builder to decide for himself (herself). Having given my disclaimer, here are some numbers.
I am seeing bolt gaps in the range of 3 to 10 thousands of an inch. Case
reveals are in the range of 0.152 to 0.160 inches at the extractor cut. The standard upper limit for new rifle excess headspace is 6 thousands of an inch. I usually see 2 to 4 thousands. I have not seen a standard for field headspace, but my experience is that IF EVERYTHING ELSE IS OK, headspace alone is not critically important as long as you are not using reloaded cases. The key thing to know about headspace and bolt gap is that if either is out of spec, you need to find out why. It could mean that the rifle has been damaged. More on that later. I am seeing bolt head recess in the range of 0.110 to 0.117 inches. These numbers are based on examination of the following Aks. SLR95, Yugo M70, Polish UF, Saiga, Russian type III, Hungarian 63/65/85 and Romanian. These are just my observations. They are not gospel. If someone else is checking these and has different numbers, please speak up.
Guacho1 Yes, you are correct. And from what I can tell, KABOOMs in AKs are extremely rare. For a good answer to that question, we need to hear from someone who has worked in a combloc armory and has inspected a large number of AKs. He might be able to tell us what the most likely problems are. I suspect that he would tell us not to be too concerned, because the AK service manual lists pages of inspections, and never once mentions headspace or bolt gap. For this reason, I am not trying to convince anyone that they need to check anything. But if they do start checking, I think we would all agree that it is vital that they understand what they found before they decide to make corrections. I think the most likely cause of problems is the guy with gauges that doesn?t fully understand the relationship of the three key dimensions: bolt gap, headspace and bolt head recess. And I hope that by the end of this thread, that problem will have been solved for those monitoring the thread.
ETA clarification: Headspace is measured between the bolt face and a reference point on the shoulder. Excess headspace is the amount of headspace that is available beyond what is needed to chamber a standard case. A few extra thousands of an inch of excess headspace is needed to allow for fouling and variances in case dimensions. Excessive headspace occurs when the excess headspace is larger than it should be. Target rifles are set with almost no excess headspace in order to maximize accuracy. Military rifles have more excess headspace to allow for ammunition variances and fouling. -------------------------Bolt head recess
Bolt head recess is the depth of the depression in the end of the bolt. It is only important if you are troubleshooting headspace problems. This is because headspace is the sum of chamber depth plus bolt gap plus bolt head recess. If your headspace is off, it could be due to any one of these factors. Bolt head recess is the least likely problem, but if you are having trouble, it needs to be eliminated as a cause to be sure.
Below are pictures of the recess in the bolt head and the gauge I use to measure it. I had to make this gauge because I could not get consistent readings with calipers. The dial indicator reads 0.200 when the gauge is placed on a flat surface. So I have to subtract its reading from 0.200 to get the bolt recess value. I know it sounds obnoxious, but that is how dial indicators work. The gauge in the picture is indicating a bolt recess of .111 (0.200 - 0.99). I checked several rifles and found the measurement to vary between 0.110 and 0.117 inches.
Be sure not to measure at the firing pin hole or the ring shaped groove at the outer part of the bolt face.
Thanks for joining the discussion. If I understand you correctly, you are questioning if there should be any bolt gap. The answer is that there has to be some gap to assure smooth and reliable functioning of the action in field conditions. I am seeing about 7 mils in the dozen or so rifles I have measured. Second, I think you are asking why, after building numerous rifles, you are hearing about it now for the first time (except maybe for G3s). The answer is that if you are only setting up rifles with prechambered barrels and premachined bolts, then it was factored in when the manufacturer chambered the barrel and machined the bolt. If you trust that the manufacturer is perfect, then you have nothing to worry about. But we are talking about demilled rifles from various countries produced over decades of time, possibly with swapped parts and battle time. Now I would agree that the odds are that you wont have any trouble. And I am not saying you have to check bolt gap (although it is very easy to check). What I am trying to do is promote a full understanding of the factors involved in case someone reading this thread happens to find out his rifle doesn?t headspace right and wants to know what to do about it. With that goal in mind, I will summarize the basic concept. If you are setting up a rifle with a prechambered barrel and premachined bolt, then the bolt recess and the chamber depth are fixed. That is, you have no way of adjusting them. Your only adjustment is movement of the barrel toward or away from the bolt. When you move the barrel in or out, the bolt gap gets smaller or bigger. If you adjust (repin) your barrel, you are affecting the bolt gap and the amount of case that is not supported by the barrel. If you have headspace problems, it is a good idea to check to see what the bolt gap is. It may give you a clue as to the
problem. Fair enough?
OOPs Sorry. Just saw your last sentence. Didnt mean to go on if you already understood. Oh well, I'll just leave it as is in case a summary would help someone else. This thread has turned into a bit of a jigsaw puzzle. --------------------------------I found this in a HK G3 armorer manual on page 5. It shows how bolt gap is used to check headspace in a G3. There is no mention of headspace gauges. I suspect that AK armories use a similar approach, but havent been able to prove it.
ETA: Looks like the image didnt come out quite right. I'll try to do better later.
-------------------------Ozzy, "bolt gap" on a G3 is not the gap between the bolt and breechface, it is the gap between the rear of the bolt and the bolt carrier. It is checked on an empty chamber as it is not a headspace related measurement. With the bolt
bottomed out against the breechface, the gap between the bolt and carrier indicates the state of wear in the roller locking mechanism. As the rollers wear the locking piece goes deeper into the bolt and the gap gets smaller.
Headspace is not normally checked on a G3 as there is really no easy way to do so. The bolt is not "locked" in the traditional sense so headspace is not too critical. ---------------------------OK, so your AK build checks out fine for headspace, but you have excessive bolt gap. What then? Only options I can see are throwing away the barrel, or pressing the barrel further back and then enlarging the chamber with a reamer, destroying any chrome lining which might be there. Or am I missing something obvious? ---------------------------Fixed the G3 image above.
I am in full agreement that it is reasonable to expect that the factories chamber and assemble the rifles correctly. As I noted previously, the AK service manual describes armorer inspections from one end to the other, but never mentions headspace. And that is the basis for my position that the bolt gap is the most important check a rebuilder can do. Bolt gap is the only headspace related dimension that the rebuilder can affect. The other dimensions (chamber depth and bolt head recess) are fixed at the factory. If I were to buy a rebuilt AK, the first thing I would do would be to check the bolt gap.
I understood that the bolt gap on a G3 is measured at a different location than on an AK. But I do not agree that the HK manual and I are incorrect in saying that G3 bolt gap is related to headspace.
If you find that your bolt gap is out of spec, the most important thing to do is to determine why. It is a good bet that it did not come from the factory that way. If it is now out of spec, then the headspace is almost certainly also out of spec. If the headspace checks OK and the bolt gap doesnt, I would suspect measurement error. Headspace and bolt gap are mathematically entertwined (functions of each other). One can not change without the other changing. If either bolt gap or headspace are out of spec, there is a good chance that the bolt, barrel and/or trunnion are damaged or worn. The first thing I would look for would be a ring in the barrel, indicating the rifle had been fired with a barrel obstruction. If that were the case, the action would have been subjected to overpressure and could be damaged. I would also inspect the locking lugs and
recesses for damage such as galling, cracking or deformation. If I had removed and reinstalled the barrel, I would suspect that the pin was driven in with the barrel slightly out of position and now the pin or barrel is galled, cut or otherwise deformed. Any damaged parts should probably be replaced. -------------------------I think you could safely determine proper bolt gap by inspecting fired cases,something you should always do with a new gun.I think in theory your concerns are valid and you will have to decide how you want to address them.In practice there does not seem to be a problem. This dim is normally treated as non-critical and in fact this is the first time I have seen it discussed. OLD SANG ----------------------------OS, Who is it that normally treats "this dim" as non critical? ----------------------------Bolt gap measurement on a G3 is not related to headspace at all. Headspace is normally measured from a point on the shoulder area of the chamber to the bottom of the casehead recess in the bolt. On a G3, this dimension is set by the depth of the chamber in the bbl. If you wanted to measure actual headspace on an assembled G3 you would have to build a depth micrometer setup to go in from the rear of the receiver and measure the protrusion of the headspace guage from the rear surface of the bbl and also measure the recess in the bolt.
What the armorer's manual is telling you to measure is the gap between the bolt and carrier. It is measured with an empty chamber, the bolt is pushed up against the back of the bbl when it is measured. The distance between the shoulder and the bolt face is not being measured in this situation. If you think it is, please explain how this is done.
The G3 bolt gap dimension is variable because the bolt rollers and their recesses in the trunnion wear with use and the locking piece which is attached to the carrier goes deeper into the bolt to compensate. Once it goes in so far that the gap disappears the self adjustment is gone. The normal cure for this according to the manual is to install oversize rollers to compensate for the wear. Again, this does not affect the distance from the boltface to shoulder.
The G3 action is often referred to as "roller locked" but this is not exactly accurate. The bolt is never locked, the actual correct terminology would be "roller delayed blowback". A straight blowback .308 would require a bolt weighing some 30+ pounds to operate safely. The rollers introduce a "mechanical disadvantage" into the equation that reduces the force the bolt
imparts to the carrier during firing. ----------------------------- The HK specified headspace check (bolt gap check) verifies acceptable shoulder to bolt face headspace as follows:
Excess headspace. If the bolt gap reaches zero, the forward position of the bolt is uncertain and the possibility exists that excess headspace is present. That is, there would be no assurance that the locking mechanism is holding the bolt head fully forward. If the minimum specified bolt gap is present, then excess headspace will not exist as long as the factory machined and checked the chamber and the bolt head recess correctly.
Inadequate headspace: Inadequate headspace results in cartridge/bolt face interference. The bolt head would not be able to fully advance, the rollers would not be fully set in place, the locking piece would be forced back by the rollers and excess gap would exist between the bolt and the bolt carrier. If the maximum specified bolt gap is not exceeded, adequate headspace exists.
Additionally, the HK specified headspace check verifies that the cartridge head is not overexposed. This is extremely important, as I noted earlier, because the potential for case wall rupture exists if the case head is overexposed.
First, if zero bolt gap exists, then the position of the bolt head is not known and the case could be allowed to back out of the chamber. Case head overexposure would result. Second, imagine that the barrel is short chambered and the case is sticking out of the barrel too far (case head overexposure). The case would hold the bolt head away from the barrel face, the locking piece would be forced back by the rollers and excess gap would exist between the bolt and the bolt carrier. If the bolt gap is within specified tolerances, then proper case head exposure is assured. --------------------------Originally Posted by ozzy the nuke The HK specified headspace check (bolt gap check) verifies acceptable shoulder to bolt face headspace as follows:
Excess headspace. If the bolt gap reaches zero, the forward position of the bolt is uncertain and the possibility exists that excess headspace is present. That is, there would be no assurance that the locking mechanism is holding the bolt head fully forward. If the minimum specified bolt gap is present, then excess headspace will not exist as long as the factory machined and checked the chamber and the bolt head recess correctly.
Which is what I said, the gap is a measurement of the locking roller engagement. "There would be no assurance that the locking mechanism is holding the bolt head fully forward". Two identically chambered G3 rifles, one has a bolt gap of .007 and the other has one of .019. Which one has a longer headspace? Neither, they are the same! One just has a little less wear on the roller mechanism than the other one.
Quote: Originally Posted by ozzy the nuke Inadequate headspace: Inadequate headspace results in cartridge/bolt face interference. The bolt head would not be able to fully advance, the rollers would not be fully set in place, the locking piece would be forced back by the rollers and excess gap would exist between the bolt and the bolt carrier. If the maximum specified bolt gap is not exceeded, adequate headspace exists. Since HK bolt gap is measured on an EMPTY chamber it wouldn't tell you much about a short chamber. AFAIK there is no published factory spec for bolt gap with either a cartridge or headspace guage in the chamber.
Quote: Originally Posted by ozzy the nuke Additionally, the HK specified headspace check verifies that the cartridge head is not overexposed. This is extremely important, as I noted earlier, because the potential for case wall rupture exists if the case head is overexposed.
First, if zero bolt gap exists, then the position of the bolt head is not known and the case could be allowed to back out of the chamber. Case head overexposure
would result. Second, imagine that the barrel is short chambered and the case is sticking out of the barrel too far (case head overexposure). The case would hold the bolt head away from the barrel face, the locking piece would be forced back by the rollers and excess gap would exist between the bolt and the bolt carrier. If the bolt gap is within specified tolerances, then proper case head exposure is assured.
Q.E.D. You're right, case head overexposure could result in a case blowing upon firing. However, this would be because the action is partially unlocked. Assuming it were possible to do so, one could experience the same thing if they were to fire a bolt action with the bolt only turned down 1/4 of the way to locked. This is essentially the case with a G3 rifle that has no bolt gap.
If bolt gap on an HK were measuring headspace, the cure for an incorrect bolt gap would be to adjust the length of the chamber. Instead, one adjusts the wear parts of the locking mechanism. ------------------------------HK measures bolt gap as a headspace check, not a headspace measurement. I recognized this distinction from the beginning. And so, I was careful to only refer to the HK headspace check as a headspace check (not a headspace measurement). And I explained why the HK bolt gap measurement procedure is a satisfactory headspace check. Meanwhile, the Kernel has explained why bolt gap measurement is not headspace measurement.
Now, I return to AKs.
AK bolt gap measurement can similarly be used to reach reasonable assurance that AK headspace is acceptable. I propose that it is safe to assume that the chamber and the bolt head are correctly machined on a demilled AK. If that is a reasonable assumption, then the only component of headspace that needs to be checked by a rebuilder is the bolt gap. If the bolt gap is within tolerance, then there is reasonable assurance that the headspace is within tolerance. Checking bolt gap requires no specialized tools. The necessary feeler gauges can be purchased at Walmart or an autoparts store for a just few dollars. My second post in this thread shows how easy it is to measure bolt gap.
The loop has been closed.
----------------------------------Having re-read your initial post, it does sound like you have inadequate headspace. This is commonly caused by installing the barrel a little too far in (5 to 10 thousands) and then managing to get the pin in anyhow. The pin has probably shaved a little off the front side of the barrel hole (actually, half hole). As a result, the barrel is sitting a little too far in toward the bolt. No big deal. The easiest solution is to pound the barrel back forward using a punch (or big bolt) with pennies or nickels to protect the chamber entrance. Do this with the pin still installed. The barrel will stop when it is fully forward against the pin, where it belongs. This may sound like a crude method, but I and others have used it successfully before. If you are lucky, the barrel will shift forward against the pin (where it belongs) and you will be able to chamber a round. Next check your ?no go? headspace using a cartridge with discs made from sticky notes on the end of the head. 3 discs equals about 6 thousands, which is the standard maximum for headspace. Not everyone agrees that a case with shims on the back of it is good enough for this purpose, but I actually consider it to be best (for reasons given in my thread GJ linked above). If the barrel doesn?t move forward, feel free to ask for better advice.
OBTW. I do not recommend redrilling the pin hole unless you are absolutely sure that it is the necessary and correct action.
