This article is intended to be a simple guide to understanding headspace for the purposes of determining whether its safe to shoot the milsurp rifle you just bought. It is not a complete, or a highly technical guide to all aspects of understanding it, nor intended as a guide to gunsmithing or rifle building. Some may find fault with the oversimplification of terms, but I'm not writing a book here, although this will be a long article, nor do I need to cover aspects more experienced hobbyists already know.
Headspace is no more than the fit of the cartridge case in the chamber of the gun, or more accurately, the fit of the cartridge in the chamber from the face of the bolt supporting the open end of the chamber and that portion of the chamber, which limits the forward movement of the cartridge case. An explanation of why it has importance to us as shooters is necessary. A gun is actually an internal combustion engine, the barrel is the cylinder, the bullet the piston, the powder is the fuel, the primer the spark plug. We dont need valves or connecting rods because that piston is not coming back to be reused. Its work is downrange. The muzzle end of the barrel is open, as is a cylinder in a car, but the other end must be closed to provide a seal.
The gas pressure in a modern cartridge used to be expressed as pounds per square inch but the more accurate term used these days is copper units of pressure. This is established in a laboratory in a special pressure test barrel. The pressure exerted by the gases squeezes a copper disc of known strength. The compression of the copper disc is measured, and the pressure of the gases is thus measured against an established scale. The terms are not quite the same but tend to be used interchangeably. The point being is that you may have superheated gas at as much as 55,000 cup trying to escape from that chamber in all directions. It's supposed to push the bullet down the bore and escape out the muzzle. If it escapes out the end in front of your face you may need a new set of eyes and some extensive plastic surgery if you survive.
You now understand the fact that the open rear of the chamber must be effectively sealed each and every time you fire a round. Most of the time, or its pretty good wont work. The bolt or breech face does not seal the open end of the chamber, it simply supports the cartridge case which does, and provides a method to feed and extract the cartridge cases. The brass cartridge case expands from the gas pressure to the sides of the chamber effectively sealing the chamber. Its held in place by the bolt or breech face against the rearward thrust. The recoil or kick you feel is the rearward thrust of the gases escaping out the muzzle plus the weight of your bullet. Once again let me emphasize you dont want that power directed at your face.
There are four common methods of establishing headspace, which I will explain separately.
The distance between the bolt-face and the edge of the rear of the barrel. More simply, the thickness of the cartridge rim that fills that gap. A good example is a British .303 Enfield chamber. If you have ever compared a fired and unfired case from an Enfield you will notice there is a great deal of expansion in the fired case. The shoulder of the case may have moved forward and it is a lot fatter than the unfired case. This was deliberate, and the chambers cut oversize so that dented, corroded, and muddy cartridges could still chamber and fire. As long as the spring or expansion of the brass case allowed for the sloppy fit and the case was supported in the rear, all was well.
A belted cartridge like a 7mm Magnum has a ring of brass at an established position on the cartridge case. The ring is not to strengthen the cartridge case from some higher pressure used in a magnum cartridge. Although the companies selling magnums want you to think so, they operate at the same high end of the pressure scale, as do other modern cartridges, the limit of which is about 55,000 cup. The working pressure of a .308 Winchester, and a .300 Winchester Magnum is the same. The ring on the case is to establish headspace. In a belted round the headspace is the distance from the bolt or breech face and the front of the ring of brass. Its a headache to a reloader because the expansion and rearward thrust changes the dimensions of the brass ring and there is no way for the reloader to re-establish the dimensions of the ring and thus control the headspace later. Case life on belted cartridges is less than a non-belted case.
Rimless Straight Wall Cartridges
A rimless straight wall case is best illustrated by a pistol cartridge like a .45 ACP or 9mm. The only method of establishing the headspace is the mouth of the cartridge case hitting the edge of the chamber in the front. The headspace is controlled only by the length of the cartridge case. This is only applied in relatively low-pressure applications like a pistol cartridge. If a reloader trims his rimless straight cases excessively before reloading he has destroyed the headspace and runs the danger of the case being driven too far into the chamber and the unsupported brass rupturing through the rear. More commonly the case head is unsupported by the breech face and the primer blows out of the rear of the cartridge case. In some cases, the firing pin will push the cartridge too far into the chamber without igniting the primer, causing a misfire.
I can tell you from experience its no fun. I didnt lose my hand but the magazine blew out the bottom of the well and the grips split in my hand. When you read about someone firing a 9mm Luger cartridge in a 9mm Largo or a .38 Super chamber, he is taking a risk on losing what he holds a fork with. The extractor may hold that case against the breech and things go well, BUT remember what we said about MOST OF THE TIME above. It is a grossly excessive headspace problem and not worth the risk.
Rimless Bottlenecked Cartridges
This type is what we usually encounter in our Milsurp Rifles, and its the most commonly used method of establishing headspace in modern rifles. The headspace here is the distance between the bolt or breech face and the center of the shoulder cut in the chamber. Its better visualized by looking at the cartridge case and thinking about the distance from the head stamp portion at the rear, to the center of the shoulder of the case. Not an exact comparison but adequate for our purpose.
In all of the applications listed above there has to be a tolerance level in manufacture. A little extra room has to be there so the bolt closes smoothly on a chambered cartridge. There are manufacturing tolerances of plus or minus a few thousands in every cartridge made, every chamber reamer made, every bolt, and every receiver. The brass cartridge case is an elastic material; under pressure it expands to fill tolerance gaps, and then shrinks or springs back slightly to allow it to be withdrawn from the chamber after firing. If the headspace, or cartridge support/fit in the chamber stays within the tolerances of the brass case to expand we have no problem. If it exceeds the capability or characteristics of the metal used in the cartridge case we have a potential massive gas leak pointed at your face.
Thats an easy one to descibe. With this condition you cant close the bolt or breech. The cartridge case is hitting that portion of the chamber or barrel that limits forward movement. Unless the chamber was cut too shallow during manufacture, the only way we get insufficient headspace is that somewhere a different bolt was placed in the gun, that reduced the tolerances below the safe level. There is no other way I can think of to give a gun insufficient headspace if the ammunition is within manufacturing specifications.
When it happens dont force the bolt closed with a rawhide mallet and blast away. The front of the chamber is cut so that the mouth of the cartridge expands to allow the bullet to begin its forward travel down the bore. The pressure developed by a given amount of powder in a cartridge case is controlled by the space it has to expand into. The bullet begins its travel long before all the powder burns and reaches full pressure, if you limit the space it has to expand into you get excessive pressure beyond the design limits of the gun, and/or the steel used to construct it. Cramming the case into the chamber usually squeezes the brass cartridge case into the end of the chamber and into the bullet not allowing it to begin its travel when the pressure is sufficient to overcome its weight and friction but before peak pressure is reached.
Visualize a firecracker in a small cardboard matchbox; the pressure is going to shatter the container because the construction material of the container is insufficient to contain the gas pressure in the space available. Now visualize a box made of the same material as the matchbox but about the size of a shoebox. Its not going to shatter the box, because the space internally allows the firecracker to expend the same amount of force in a larger area. The pressure exerted against the sides of the box is actually reduced because the same amount of pressure has a greater area to expand into. Get the picture? The bullet moving forward at the proper moment expands the area the gas has to exert its energy and keeps it within your barrel and receivers strength limits.
This is what we usually worry about when we get that new milsurp. While there is no way to get insufficient headspace except swapping bolts, not so with excessive headspace. While it is most commonly encountered by bolt swaps sometime before we bought the gun, its not limited to that. Simple wear will do, as will other factors such as soft metal in bolt locking lugs or receiver lug recesses being compressed during repeated firing. Firing over pressure loads will speed up that process considerably.
In this condition there is an excessive distance between the bolt or breech face and the cartridge shoulder in your Milsurp. This will allow the primer to back out of its pressed fit in the cartridge case and leak gas rearward, or much worse, the rear portion of the cartridge case to back out of the chamber and rupture giving a catastrophic gas leak to the rear straight at you. In between we have a partial case head separation, which also does nothing for the ballistics, your nerves or scores on later range trips, or your longevity. Worse than that it may damage your new rifle.
How do I measure headspace in my gun?
A set of gauges is used to measure the upper and lower limits, as well as a geeze this thing is unsafe to fire gauge. They are too expensive for the casual milsurp collector and not really necessary in most cases. I confess I dont own a set, and when I determine they are needed, I borrow a friends or its a trip to the gunsmith in some calibers. They are precision instruments and must be treated as such. Follow the manufacturers instructions on their use as it may vary by manufacturer, and most require the extractor to be removed prior to use. The only truely safe answer I or anyone can give you when there is a headspace question is take it to a gunsmith. Even using the gauges takes some interpetation. The methods I and other experienced shooters use works fine for me, but, it requires experience, technique, and a few sphincter-tightening mishaps in your past. Im not going to tell you all my idiotic blunders lest you stop reading my articles or come up with a nickname I wouldnt be proud of. I used to shoot with a cousin who was right handed but shot with his left eye. He lost the right in a gun accident and it was a constant reminder to me to exercise safety.
These are some techniques I use. I list them for what ever safety value they have BUT, The ONLY SAFE THING to do is take it to a gunsmith. The first thing I do with a new gun is check the serial number of the bolt and receiver, if they match, even if re-marked or electro-penciled I breathe a sigh of relief, but I dont stop there. Next I get a fired but resized case and add two layers of masking tape (the thick kind, .003, not the cheap Home Depot "tissue tape") to the head and try to close the bolt. Make sure the tape on the cartridge head is trimmed off and doesn't overlap the edges, onto the sides of the case. If I feel a lot of resistance, or it doesnt want to close without force I'm a happy man without that 3rd beer.
I also use some feeler gauge intended for spark plug gapping in various thicknesses cut round to fit the cartridge case head. Its easy to stick it where you want it to stay with some thick grease. The grease in the little containers intended to be stored in the butt stock of your Garand works very well to hold the feeler gauge together and on the case head. The fired cartridge case was fired in a gun with known headspace and was not re-sized completely. This gives me a margin of error I'm usually comfortable with. I compare the amount of feeler gauge used until the bolt wont close with other guns known to be safe and keep the data in a small notebook. If its the same or less than one I know is OK I believe the gun is safe to test fire. Obviously you need several or at least more than one gun of known headspace and the control of that single cartridge case as a comparison. This technique will not be of any use to most of you, especially if you dont own multiple guns in the same caliber. You can understand now why I am a bit reluctant to share it. It does have some value, but only in similar circumstances. I'm comfortable using it but cant swear to its value in your gun unless I'm doing the interpreting of the feel on that bolt closing myself.
Every new rifle I buy regardless of how it comes through the above is completely disassembled, cleaned, and inspected before I fire it. See my article on cleaning and inspection listed elsewhere. It's then usually strapped down to an old tire and fired with a long string on the trigger at least 3 times. The ammo is always the same test stuff used in many other guns with no problems or unusual effects. The fired cases are very carefully inspected for visual evidence like a bulge about 3/8 of an inch in front of the extractor groove. If I see that in a military cartridge case or the gun fails any of the tests above its going to the gunsmith.
This test should be done as a safety measure even if the gun has good headspace. Your WW I or WW II vintage rifle has seen a lot of service and who knows if some home gunsmith decided he wanted to "heat treat" the bolt before you bought it. He may have gotten it so hard it shatters on the first shot. You have no way to x-ray, magnaflux etc. to detect a crack or weak spot you can not see. I once owned an Arisake I never fired, and gave it to a friend as a gift. He wanted a sporter and pulled the barrel without firing the rifle and later handed me the barrel and a magnafying glass. The barrel had a hairline crack on the underside hidden by the stock. Test it first and then you wont shake so bad the first time you fire it from your shoulder.
So, its a little off, what can I do?
Well its time for the gauges if you know its off. There are 3, GO meaning its not too tight: if it closes on this one. NO GO meaning if it closes without resistance its a little loose, and FIELD if it closes on this one, its dangerous. If its way out of specs you need professional help and maybe a new barrel or bolt. I dont have space to go into the gunsmithing techniques to fix it here. If its a little loose but not yet dangerous I believe it's safe to fire with thick cased military cases if it passed my other tests.
Headspace can be adjusted by hand loading within limits. A case is fire formed in the chamber of the gun at low pressure, and then only the neck portion holding the bullet is resized. This uses the brass case formed in that gun to control the distance between breech and shoulder and makes it safe to fire with subsequent reloads at normal pressures. Its of value if it wasnt unsafe to fire to begin with. Primarily it gives an added measure of safety not present with factory spec cartridge cases and extends the brass life of cases fired in that particular rifle.
How common is it to find bad headspace
Well I counted up my rifles the other day and there are 63 of them right now. Countless others have passed through my hands. Its pretty hard for a pistol to change headspace so I wont bother with them. Conservatively lets say I have had intimate acquaintance with maybe 150 rifles. In that time I have found one Savage Sporter, one Turk, one VZ24, and one Krag with headspace far enough out that they were not safe to fire factory ammo. The Krag was purchased knowing the headspace was bad but it was a rare model I decided I needed to have. Even it will be restored to use at some point. In a word, it's not likely you will get a bad one. We need to be aware of what headspace is, and what to watch for, but not live in fear that every potential purchase is a bomb or likely to need expensive repairs. I hope this helps you all in pursuing your hobby. Good luck finding the one model you are searching for cheap, and Happy Shooting. Jeff7mm
Curio and Relic Firearms Forum, Swedish Rifles page
"the right of the people to keep and bear arms shall not be infringed...." Folks, that is in ENGLISH, and "INTERPRETATION" is not required!
"the right of the people to keep and bear arms shall not be infringed...." Folks, that is in ENGLISH, and "INTERPRETATION" is not required!