Gun Care

Headspace and Why It Is Important

Illustration of four cartridges showing the headspae measurement for each type

An important concept called “headspace” is often overlooked and unknown to many shooters who don’t realize how much they don’t know. I never realized how few shooters have even heard of, or been concerned about, headspace. That fact was pressed home when I was at the range one day and another shooter was having all sorts of issues with a new AR-type rifle.

The rifle would not fire more than one round. When it did fire, it would not cycle and would not go into battery. Knowing I am an instructor, he asked me my opinion. I said, he should return it to the manufacturer. He then said something that should not have surprised me. “I made it,” he declared.

Illustration of a .45 ACP rimless cartridge seated in a chamber.
A .45 ACP rimless cartridge seated in a chamber. The case mouth is larger than the bullet diameter and rests against the front of the chamber, thereby correctly seating the cartridge.

I asked, “What do you mean, I made it? Did you manufacture the parts?” He clarified that he bought the parts and assembled them. In my naïveté, I asked who’s upper he used. He again proudly stated, “I put that together from parts also.” The first thing that leaped into my mind was, of course, the headspace.

I asked if the headspace was set correctly. That’s when the conversation took an interesting turn. He had the look of a deer in the headlights, a kid caught with his hand in the cookie jar, or your best friend caught in bed with your wife… HUH? “What is headspace?” he uttered feebly. At that moment, I told him to clear the weapon, put it in the case, and leave the range. He was not to return until the headspace was set properly.

How could someone be so ignorant as to think they could assemble a firearm and not even have heard of the term, headspace? Now that’s off my chest. I got some splanin to do, Lucy!

What Is Headspace?

Simply put, headspace is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge. As an example, in the accompanying illustrations, we have cutaway views of a .22 rimfire, bottlenecked rifle cartridge, .45 ACP, and a belted magnum cartridge shown how they should appear in a chamber. On the .45, notice that the cartridge case is a bit larger than the bullet by the thickness of the brass case, and the chamber is shaped so that it fits the case correctly. Of course, depending on the shape of the cartridge, the headspace could be different for different firearms as seen in the illustrations.

Illustration of a the rim of a centerfire, rimmed rifle cartridge
This is the rim of a centerfire, rimmed rifle cartridge.

Most of the earliest metallic cartridges were rimmed. This meant, they had a rim at the base of the cartridge that was larger (refer to the illustration of the .22 LR) than the diameter of the cartridge case. When this type of cartridge is pushed into a firearm’s chamber, the rim positions the cartridge correctly and prevents it from going too far into the chamber.

Because of that design feature, a rimmed cartridge’s headspace is determined by the thickness of the rim. The most popular cartridge today, the .22 Long Rifle rimfire cartridge is an example of a rimmed cartridge. It should be noted that it is common today to find that calibers intended for use in revolvers have rims that establish their headspace.

That means that most modern cartridge designs are of the rimless type. i.e., the diameter of the rim is the same or smaller (rebated) than the diameter of the case body. Modern cartridges also have other ways to ensure proper seating of the bullet in the chamber. One example would be pistol cartridges where the case diameter is slightly larger than the bullet and the chamber has a shoulder that the case mouth rests against as in the illustration of the .45 ACP.

On the base of a rimless cartridge, you’ll notice that the rim does not extend beyond the width of the case.

However, it must be mentioned that there are other rimless cartridge designs, such as bottlenecked cartridges (which are mostly used in rifles), the shape of the firearm’s chamber is correspondingly tapered to achieve proper headspace. That is because the headspace on a bottlenecked rimless cartridge is the distance from the bolt face to the tapered section or shoulder.

In the case of belted cartridges, a design introduced by the British firm of Holland & Holland on its proprietary calibers (such as the .300 Holland & Holland Magnum), where the chamber is shaped to seat the forward face of the belt.

Illustration of a shouldered rifle case showing headspace
Different cartridges result in headspace measurements at different points on the cartridge/chamber.

When the .300 H&H Magnum was first being developed, it was not possible to provide proper headspace for a cartridge with a shallow shoulder design. Holland’s solution was to add a belt around the cartridge body. This is similar in function to the rim of a rimmed cartridge, but gave a long enough surface to allow the cartridges to fit side-by-side in a magazine without interfering with the feeding of those cartridges. This design feature was later used on other magnum rifle cartridges designed by others as well.

As previously mentioned, headspace is the distance between the bolt face and a point in the chamber that prevents further forward movement of a cartridge. Depending on the firearm and the shape of the cartridge, this distance can be different for different cartridge types, as the images show.

Improper Headspacing

Now, let’s look at what happens when the headspace is not correctly set, as in the anecdote mentioned earlier. There are two possible scenarios we must look at. The first is excessive headspace. Excessive headspace is best described as the condition where there is extra space between the bolt face and the cartridge in the chamber.

When that condition exists, the firing pin hits the cartridge and it will move forward into the chamber before detonating. When the propellant does ignite, the walls of the cartridge expand due to pressure and firmly stick to the walls of the chamber, preventing rearward motion of the cartridge. When that happens the thicker base of the cartridge will move backward.


This happens because there is a gap between the cartridge and the bolt face, and this will cause the walls of the cartridge to stretch. If the stretching is too much, the walls of the cartridge could rupture, release hot gases into the action, and potentially spray brass fragments out from the action of the firearm. In turn, this could be hazardous to the shooter or anyone standing next to him.

The second situation is created when there is insufficient headspace in the chamber, or too little space exists between the bolt and the cartridge. In this condition, the back of the cartridge will stick out and the bolt will not be able to close fully on the loaded cartridge. The user will not be able to properly operate the firearm when this happens.

 belt of a belted magnum cartridge
On the belt of a belted magnum cartridge, you’ll notice that the rim is wider than the case but not wider than the belt.

If the user were to force the bolt to close on the cartridge, the bullet will be pushed tightly into the case neck. If the firearm is fired, this will cause excessive pressure to build up inside the cartridge case, leading to hot gases coming out of the cartridge’s primer pocket — with similar results to excessive spacing. In a worst-case scenario, the excessive pressure could cause the action to rupture and cause damage to the gun and its user.

So, the next question to ask is how to determine whether a firearm has proper headspace? We can do that by using a set of headspace gauges. These are measuring instruments that are precisely machined to the SAAMI, CIP, or military standards for a specific cartridge or caliber. Typically, headspace instruments are made of heat-treated steel and are machined to tolerances below 0.001 inches or so. They are made in various calibers and sold at reasonable prices. Typically, there is a “Go” gauge, a “No-go” gauge, and for military specification rifles, a “Field” gauge.

Go Gauge

The bolt on a firearm must be able to close with no resistance when a Go gauge is inserted into the chamber. This signifies that the firearm is able to meet the minimum length specification for that particular cartridge. If the bolt does not close with the Go gauge inserted, the firearm has insufficient headspace. Another possible cause could be a dirty chamber or bolt face. The accumulated dirt may be thick enough to prevent the bolt from closing on the gauge. However, if the firearm is clean and the bolt still does not close on the Go gauge, it must be taken to a competent gunsmith for adjustments.

If a firearm successfully closes on the Go gauge, at a minimum, the firearm has sufficient headspace. However, it may still have excessive headspace. That can be determined with No-go gauge.

two headspace cartridges
Headspace gauges – The top one is a No-go gauge, and the bottom is a Go gauge.

No-Go Gauge

A new (or overhauled) firearm must not be able to close on a No-go gauge. If the bolt closes successfully on a No-go gauge, this means the firearm has excessive headspace and there is a risk of cartridge cases rupturing inside the chamber. If the firearm is new or recently repaired, it should be returned to the manufacturer immediately.

A used firearm may be able to close on a No-go gauge, due to wear of the bolt and chamber surfaces. This means it should probably go to a gunsmith for repair soon — it may be possible to fire new factory ammunition in it until then, but reloaded ammunition is probably a bad idea. The firearm may malfunction on slightly out-of-spec cartridges. Here’s where the test with the third gauge comes in (especially for firearms built to military specifications).

Field Gauge

The bolt of any firearm, whether old or new, should not be able to close on a Field gauge. A bolt that closes on No-go, but not on a Field gauge, may be considered close to being unsafe, but may work on new cartridges. Likely, it should be sent to a gunsmith to have the headspace reset. However, if the bolt closes on a Field gauge as well, then it is not safe to fire and should be sent for repairs immediately.

Illustration of a semi-rimmed cartridge
Semi-rimmed cartridge — The rim is slightly wider than the diameter of the case.

Some calibers have a fourth gauge, known as a Field II gauge, for which the bolt should never lock on. This type of gauge is only used by certain rifles. For example, Colt uses it to reject M-16 rifles. It must be noted that gauges are usually manufactured to either SAAMI, CIP, or military standards, and therefore may have different dimensions, even for the same caliber cartridge. Therefore, it may be possible that a rifle manufactured to NATO specifications, may lock on a No-go gauge built to SAAMI specifications, but correctly not lock on a No-go gauge built to NATO specifications.

This is because military weapons are generally designed to operate with wider tolerances and military ammunition cases are generally thicker than commercial ammunition. That means they can tolerate more stretching without rupturing. Therefore, a military firearm may fail the test using SAAMI gauges, but still be deemed safe to fire per the military specification gauges. However, if it passes using SAAMI gauges, it is very likely to work correctly.

These gauges are relatively inexpensive. If you fancy yourself a firearms enthusiast and gun crank, do yourself a favor and get a set of gauges and learn how to use them for peace of mind.

Do you own a set of headspace gauges? Have you identified problems with one of your guns using headspace gauges? Do you have a gun you should check with a set of gauges? Share your answers in the comment section.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (40)

  1. I agree with Flyerbob. Was the shooter justly told to not use the gun anymore? Yes. But a quick explanation of the unsafe condition, quite possibly leading to a catastrophic failure, should have been forthcoming instead of arrogance and condescension; belittling due to being uneducated. Maybe something like, “You may have a problem with what is called ‘headspace’ in your rifle, which will lead to failure and injury if it is not set properly. If you don’t know what it is and how to set it then find a gunsmith who can!”
    People learn from and appreciate education. Not being bullied by arrogant, harassing overlords!
    The guy probably has friends who also build their own but don’t know and educating the one person can have a cascade effect of teaching others!

  2. I am a government contractor performing Gunsmith repairs for our men & women serving. And pior to that I served as a Armament Small Arms Repairer/aka Gunsmith for 20+ years. I’ve always been concerned for people who do their own personal builds. Head space is key, and also having the proper tools to perform the build is just as important. As we say in the military, “Saftey first.” My recommendations to people that want to do their own personal builds or repairs. Don’t just watch YouTube, I’ve seen people talk about their builds on video and could point out discrepancies. Do yourselves a favor get manuals, poper tools, and proper training from a reputable Gunsmithing Course. Even after the many years of working on fire Arms, I still don’t consider myself an expert. I know things and if I don’t, I rely on a Technical Manual. Remember, knowledge is power. “Saftey First!!”

  3. I agree with Flyer Bob as well. Leave the arrogance at home. Helping people will possibly save shooters and re-loaders from physical injury, but also their feelings and negativity toward the shooting community. Thank you for reading my post. Be safe!

  4. I’ve read you can convert a “go” gauge into a “no-go” gauge by adding one (1) thickness of Scotch tape to the case’s base (adds 0.001″ in length. Then, of course, you must remove the single layer of tape to revert to the “go” gauge’. What do you think about this as a solution? Would adding a 2nd layer of tape (0.002″) produce a “field gauge”?

  5. I purchased all three gauges for my AR style .308 build. Yes I’m a first time builder. I knew I’d have to check headspace on my build, but didn’t know why. Thank you for the reasons. The question that comes up to me, is how hard will it be to find a gunsmith willing to look at much less work on a home built firearm.

  6. Very good information. I’m going to start a collection of necessary gauges for my favorite calibers now, because I do buy used firearms. This is a lot better than taking it out for a “test run” and then inspecting the spent cartridges for “bad signs”!

  7. Outstanding article with the right amount of detailed information, and well balanced with technical backup. I know more about the subject than I did before. Thank you.

  8. Fantastic artical on a detail many gun owners know little about, unless they are building a firearm or reloading. I reload, mostly rimmed cartridges (.357/.44Mag/38-55/30-30; but also some 8mm Mauser and 30-06). But I didn’t know all of it and what I find missing, only for the mental exersise, is how do you fix it (there are no screw adjustments); does the firearm need to be re machined or adjusted in another way (Gunsmith work). I may not want to do open heart surgery, but I would like to know some of the basics of what the surgeon is going to do, same with this process/repair.

  9. Great article! I have built a bunch of ARs over the years from parts and 80% lowers and never had any kind of issue. That is, until recently. I replaced a worn-out barrel in one of my ARs with a well known (and expensive) brand. Nothing would chamber properly – neither factory nor my re-loads. I returned the barrel to the manufacturer and after a looong time, I got it back. They confirmed a minor defect and corrected it (tolerances too tight). If I had these gauges and the knowledge from this article, I could have communicated much better with the manufacturer to solve the problem. I just did a thorough search of CTD’s website and guess what? They don’t sell the gauges!!! Dagnabbit!

  10. I think this article barely scratches the surface on what people need to know about headspace. If you’re trying to scare people and leave them in the dark with only the thought of a gunsmith to rely on, then you have succeeded. You keep talking about the same two cartridges and I think most people don’t have those cartridges. And when it comes to headspace devices you only show and explain one. Also, if the bullet doesn’t pass the headspace test what can be done about it? Rebore the entire chamber or shave some off the barrel? Both can be labor intensive, and proper accuracy would be important. Can I do this myself and how would it be done is important to firearm builders who are newbies. Pictures of the other headspace devices and some instruction on how to change the headspace using a mill or drill press might be helpful, and especially if you have a warning about doing headspace oneself. Repetitious pictures of cartridges and remarks about ‘go, no go’ doesn’t quite cut the very important cake of headspace.

  11. Headspace is very important, and should be well understood by shooters, sportsmen and everyone using a firearm. Having made custom rifles for many years, I had all type of chambering reamers, headspace gages and crocus cloth to polish the chambers to a mirror finish.

    There are all types of material on headspace and how to check it. Brownell’s is a great place to find tools and instructions.

  12. I agree with Flyerbob. I would not return to the range if I was him. He could have just explained what was wrong. It’s just smug elitism under the guise of pretending to help people

  13. Your article was perfect in the way you described what really is head space and clarified the difference.
    Thank you for the diagrams as well .

  14. I have built multiple AR from parts and swapped barrels and bolts on countless uppers and use the “Go” and Field gauges to check headspace when I do. The tolerances in the AR -15 components (chambered barrels with extension and bolts)available make this check almost seem unnecessary as I have not had one fail in a new build.

    The question you should have asked was did he also build his own ammo as well. Cheap once fired SAW brass that is not fully re sized will cause the issues you described way more often than a headspace issue. Occam’s Razor. This simplest explanation is the most likely.

  15. NICE ARTICLE!!! This subject has always been a little cloudy in its definition for me, until now. Thank you. For me it has been so cloudy, I thought only necked cartridges had head space requirements. Very simple explanation and a case where pictures are, indeed, worth a thousand words. As for people being asked to leave the range for unsafe conditions, I have zero problem with that. We can all survive a little redness, and humility, however we may not survive a catastrophic firearm failure. Now that I understand head space more throughly now, one thing I HIGHLY recommend, especially on necked cartridges, is to REALLY clean the portion of the chamber where the NECKED part of the cartridge is, and the bullet leaving the brass, leaves a ring of fouling. Bore brushes are too small in diameter for this, and most CHAMBER BRUSHES do not consider this area, so I recommend stepping up a size or two in a bore brush, to clean this small section of the chamber, using a fixed cleaning rod so it can twist the brush in the chamber.

  16. WoW! This opened up an area I gave little thought. I always learn from this writers’ articles which stimulate mostly helpful comments from readers. This article is extremely detailed and beautifully illustrated to assist me in understanding this important topic.
    In this era many have assembled their own AR’s from various sourced parts, it illuminates the importance of headspace with owner assembled firearms and those of us that buy remanufactured ammo. This writer possibly saved many uninformed shooters from injury. The article will reach readers who will take the step of having a gunsmith check their firearm to make sure it is safe.
    Those who smugly leave snarky comments are the ones who “know it all”. They can’t possibly learn anything from anyone. They just want everyone to think they are smarter than the writer.

  17. Asking the guy to leave the range may appear harsh, but a gun with bad headspacing is a safety risk, both to the shooter and to anyone nearby. The range officer’s job is to keep things safe. Fixing bad headspacing is a gunsmithing job.

  18. Another sterling explanation by Ed. I suspect the average gun owner has no clue about this aspect of gun ownership.

  19. The Doctor is in!

    Mr. LaPorte – your concise writing, ballistic insights, smoking hot topics, great graphics, I could go on are why I come back to this website more time that not. Each post a worthy chapter to a book. You are the Harvey Pennick of firearms (for the golfers out there.) Cheaper than dirt – are you paying this guy?

    Also, presume you are a vet. Thank you for your service on and off the field.

    Cheers brother, all.

  20. Nice, simple and straightforward tutorial on a vital ordnance topic that most shooters cannot really grasp easily or think they need to understand.

    Fortunately, the industry has done a good job of building weapons and ammunition so that most individuals can operate their weapons safely for the most part not having to understand the engineering and craftsmanship behind the successful weapons operation.

    Understanding a multitude topics such as this one separate the weekend and ad hoc shooter from a mere operator to something closer to an entry level craftsman.

    Unfortunately the NRA does not require Range Safety Officers to understand this topic thoroughly.

  21. The firing pin hits the primer, igniting the charge. The first item to move backwards is the primer itself. After all, it’s just a press fit. Increasing pressure, the case expands to grip the chamber, and the head moves back, reseating the primer. If the head moves beyond the ‘elastic limit’ of the brass, a shiny ring can/will occur just forward of the case head.

    SMLE owners. The removeable nose piece on the bolt was made in three different lengths. They are not interchangeable. (This allowed for wider tolerances on the bolt and receiver during manufacture.) If you see that the brass is stretched, then the nose piece is the first item to be looked at. As pointed out above, excessive headspace is bloody dangerous.

  22. Anyone who’s ever had to either run or be responsible for an M2HB Browning caliber 50 machine gun knows the term “Headspace and Timing.” I was an Infantry Captain.

  23. Many people can do things, but don’t know enough to do them safely. They don’t know what they don’t know. I understand the need to intervene to protect others, but at least let them know why, and point them to resources who can help assess or fix the problem.

  24. Flyerbob
    I am supportive of the novice.

    But if they have a dangerous non functional weapon off the range they do.

  25. .

    Ed did what I would have done. A manufacturers responsibility? Sure but re read. The guy with the AR had built it himself. I think Ed may have saved the fellow the use of his eyes!

    Another word on head space- there were a number of shoddy conversions of 7.5 French to .308 and various calibers to .308 a number of years ago. They were poorly done, dangerous, and hopelessly in accurate.
    Kuby- the NRA doesnt cover headspace. If you go to gunsmith school you will learn. Building a rifle or delving into conversions or handloading demands an understanding of headspace. Some have a lot of all around knowledge. Thats Ed

  26. Most AR barrels and bolts are machined to close enough tolerances that even a kit or random parts build should be no trouble, but there are exceptions. I know because I have encountered them. If the GO gauge doesn’t go in an AR barrel, the most practical way to correct poor headspace is to use a reamer. If the barrel is marked 5.56, use 5.56 gauges and reamers rather than .223. If one does not have the necessary skill and access to the proper tools, an alternative is to mix and match bolt heads and barrels until proper headspace is achieved. Shimming the barrel to increase headspace will introduce a host of new problems, so I can’t recommend it. If the problem is excess headspace (NO-GO gauge goes), a reamer is of no use. Get a new barrel. Do not trim the bolt locking lugs or the barrel, for this only introduces more safety and reliability issues. Unless a bolt can be located that corrects the excess headspace, such barrels should be trashed.

  27. I see my other comment finally showed up.

    I encourage everyone to be supportive of the novice, the student. Kindness is in order

  28. Interesting. It seems making any comment that could provide any critique (or chance to learn and improve), are censored. This seems to have become normal these days – on the left, right, everywhere.
    “At that moment, I told him to clear the weapon, put it in the case, and leave the range. He was not to return until the headspace was set properly.’. As an educator myself, frankly I can say now that the statement is abusive and harassing to the student.
    Therefore, this comment will probably ALSO be deleted along with my prior one.

    1. We do not censor comments, sometimes they just get caught up in the review process and spam filter. Glad to see everything is working properly for you now.

  29. Great info, thanks! I knew about Go / No Go gauges, but the info on the Field gauges was new to me. Definitely keeping this article.

  30. This is such an overlooked area for shooting- ESPECIALLY with the “home built” AR crowd.

    Hunting rifles are not immune, though.

    I once had a test rifle that would not chamber a round from 2 different types of factory ammo.
    I did a bit of a “cheat,” as I didn’t have a headspace gauge.l took an empty case and tried to chamber it, then resized the brass “extra tight”. I was able to FORCE the bolt closed but could see a mark on the case that told me the headspace was off.
    Long story short, they sent a new rifle immediately and I sent the other back.

  31. absolutely essential reading. Print, file, and consult this article often. Learn more if you assemble rifles!!!!! Read and re-read and this knowledge will save you grief work and money.

  32. You imply that improper head space is so critical to the point of kicking a person off the range!@#$%^.

    But what you FAIL to state is HOW to adjust headspace? I see ‘headspace’ as a function of the barrel machining that is a manufactures issue. Due to headspace being a function of barrel machine – why would shooters need gauges? As barrel’s are made of steal/alloys – ammo is brass/aluminum – the barrel wear factor is negligible with the friction of ammo metals.

    I see no where in this article a concern of caked burnt powder compromising headspace.

    I fail to understand your perceived headspace crisis.

    I do not remember ‘headspace’ ever being covered in any of the NRA training courses – of which I’m a certified Instructor of eight disciplines.

  33. Thank you for the timely explanation of headspace. I had forgotten much of what is covered. Since I have civilian and military weapons,very good information

  34. I realized what keep bothering me about these posts. Why is it that all the ‘experts’ in shooting activities seem so arrogant, condescending, supercilious, and rude to beginners or those who are inexperienced?
    Maybe that has changed for pretty women, who are often also treated in a condescending manner, but with a great deal of interest by the generally male population.
    This model of academics, teaching has long ago been rejected, considered abusive and bullying in most other fields of instruction.

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