This illustration shows headspace measurement for the popular .308 Winchester cartridge, which headspaces on the shoulder. Image copyright 2015 Ultimate Reloader.
In this Brownells Tech Tip Video, Brownells gun tech Steve Ostrem explains what headspace is and why it’s one of the most critical measurements for nearly all firearms. Even if you’re an experienced rifle shooter, it’s worth watching this video to refresh your understanding of headspace measurements, and the correct use of “GO” and “NO-GO” gauges.
Headspace Definition
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56x45mm NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the forward rim of the cartridge. If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed and the cartridge case may rupture, possibly damaging the firearm and injuring the shooter. (Source: Wikipedia)
Problems Caused by Too Much Headspace
Excessive headspace issues can include: light primer strikes, failure to fire, bulged/blown cases, case separations, split shoulders, or unseated primers after firing. Case ruptures caused by excessive headspace can lead to catastrophic failures causing serious injury. That is why headspace is such an important measurement.
Problems Cause by Too Little Headspace
Insufficent (or excessively tight) headspace can prevent the firearm from going into battery, resulting in failure to fire or deformation of the cartridge case. Various feeding and functioning problems can be caused by cases with too little headspace, even if a round can be chambered (with effort).
Headspace Gauges
Headspace is measured with a set of two headspace gauges: a “Go” gauge, and a “No-Go” gauge. Headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a “Go” and a “No-Go” gauge are required for a gunsmith to headspace a firearm properly. A third gauge, the “Field” gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the “factory specs” measured by the “Go” and “No-Go” gauges. A bolt that closes on “No-Go” but not on “Field” is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec. (Source: Wikipedia)
You need to understand Headspace Gauges and how to use them. There are “Go”, “No-Go”, and “Field” gauges. The “Field” is actually the longest. Brownells explains: “The GO gauge corresponds to the SAAMI minimum chamber length, while the FIELD gauge usually matches the maximum chamber depth, or slightly less. NO-GO gauges are an intermediate length between minimum and maximum, that, technically, is a voluntary dimension. A firearm that closes on a NO-GO gauge and does not close on a FIELD gauge may not give good accuracy and may have very short cartridge case life [.]”
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How and Why to Create a Dummy Round
When you have a new custom rifle built, or a new barrel fitted to an existing rifle, it makes sense to create a dummy round. This should have your preferred brass and bullet types, with the bullet positioned at optimal seating depth. A proper dummy round helps the gunsmith set the freebore correctly for your cartridge, and also ensure the proper chamber dimensions.
Respected machinist, tool-maker, and gunsmith Greg Tannel of Gre-Tan Rifles explains: “I use the dummy round as a gauge to finish cut the neck diameter and throat length and diameter so you have [optimal] clearance on the loaded neck and the ogive of the bullet just touches the rifling.” He recommends setting bullet so the full diameter is just forward of the case’s neck-shoulder junction. “From there”, Greg says, “I can build you the chamber you want… with all the proper clearances”.
Greg Tannel has created a very helpful video showing how to create a dummy round. Greg explains how to measure and assemble the dummy and how it will be used during the barrel chambering process. Greg notes — the dummy round should have NO Primer and No powder. We strongly recommend that every rifle shooter watch this video. Even if you won’t need a new barrel any time soon, you can learn important things about freebore, leade, and chamber geometry.
Must Watch Video — This has been viewed over 790,000 times on YouTube:
This has been a very popular video, with 782,000 views! Here are actual YouTube comments:
That is the best explanation I’ve ever seen. Thank you sir. — P. Pablo
Nice video. You do a very good job of making this easy for new reloaders to understand. I sure wish things like this were available when I started reloading and having custom rifles built. Once again, great job, and your work speaks for itself. — Brandon K.
Beautiful job explaining chambering clearances. — D. Giorgi
Another Cool Tool — The Stub Gauge
When you have your gunsmith chamber your barrel, you can also have him create a Stub Gauge, i.e. a cast-off barrel section chambered like your actual barrel. The stub gauge lets you measure the original length to lands and freebore when your barrel was new. This gives you a baseline to accurately assess how far your throat erodes with use. Of course, as the throat wears, to get true length-to-lands dimension, you need take your measurement using your actual barrel. The barrel stub gauge helps you set the initial bullet seating depth. Seating depth is then adjusted accordingly, based on observed throat erosion, or your preferred seating depth.
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How and Why to Create a Dummy Round
When you have a new custom rifle built, or a new barrel fitted to an existing rifle, it makes sense to create a dummy round. This should have your preferred brass and bullet types, with the bullet positioned at optimal seating depth. A proper dummy round helps the gunsmith set the freebore correctly for your cartridge, and also ensure the proper chamber dimensions.
Respected machinist, tool-maker, and gunsmith Greg Tannel of Gre-Tan Rifles explains: “I use the dummy round as a gauge to finish cut the neck diameter and throat length and diameter so you have [optimal] clearance on the loaded neck and the ogive of the bullet just touches the rifling.” He recommends setting bullet so the full diameter is just forward of the case’s neck-shoulder junction. “From there”, Greg says, “I can build you the chamber you want… with all the proper clearances”.
Greg Tannel has created a very helpful video showing how to create a dummy round. Greg explains how to measure and assemble the dummy and how it will be used during the barrel chambering process. Greg notes — the dummy round should have NO Primer and NO powder. We strongly recommend that every rifle shooter watch this video. Even if you won’t need a new barrel any time soon, you can learn important things about freebore, leade, and chamber geometry.
Must Watch Video — This has been viewed over 772,000 times on YouTube:
This has been a very popular video, with 772,000 views! Here are actual YouTube comments:
That is the best explanation I’ve ever seen. Thank you sir. — P. Pablo
Nice video. You do a very good job of making this easy for new reloaders to understand. I sure wish things like this were available when I started reloading and having custom rifles built. Once again, great job, and your work speaks for itself. — Brandon K.
Beautiful job explaining chambering clearances. — D. Giorgi
Another Cool Tool — The Stub Gauge
When you have your gunsmith chamber your barrel, you can also have him create a Stub Gauge, i.e. a cast-off barrel section chambered like your actual barrel. The stub gauge lets you measure the original length to lands and freebore when your barrel was new. This gives you a baseline to accurately assess how far your throat erodes with use. Of course, as the throat wears, to get true length-to-lands dimension, you need take your measurement using your actual barrel. The barrel stub gauge helps you set the initial bullet seating depth. Seating depth is then adjusted accordingly, based on observed throat erosion, or your preferred seating depth.
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This illustration shows headspace measurement for the popular .308 Winchester cartridge, which headspaces on the shoulder. Image copyright 2015 Ultimate Reloader.
In this Brownells Tech Tip Video, Brownells gun tech Steve Ostrem explains what headspace is and why it’s one of the most critical measurements for nearly all firearms. Even if you’re an experienced rifle shooter, it’s worth watching this video to refresh your understanding of headspace measurements, and the correct use of “GO” and “NO-GO” gauges.
Headspace Definition
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56x45mm NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the forward rim of the cartridge. If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed and the cartridge case may rupture, possibly damaging the firearm and injuring the shooter. (Source: Wikipedia)
Problems Caused by Too Much Headspace
Excessive headspace issues can include: light primer strikes, failure to fire, bulged/blown cases, case separations, split shoulders, or unseated primers after firing. Case ruptures caused by excessive headspace can lead to catastrophic failures causing serious injury. That is why headspace is such an important measurement.
Problems Cause by Too Little Headspace
Insufficent (or excessively tight) headspace can prevent the firearm from going into battery, resulting in failure to fire or deformation of the cartridge case. Various feeding and functioning problems can be caused by cases with too little headspace, even if a round can be chambered (with effort).
Headspace Gauges
Headspace is measured with a set of two headspace gauges: a “Go” gauge, and a “No-Go” gauge. Headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a “Go” and a “No-Go” gauge are required for a gunsmith to headspace a firearm properly. A third gauge, the “Field” gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the “factory specs” measured by the “Go” and “No-Go” gauges. A bolt that closes on “No-Go” but not on “Field” is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec. (Source: Wikipedia)
To learn more, read Brownell’s article Headspace Gauges and How to Use Them. Among other things, this explains the relative lengths of “Go”, “No-Go”, and “Field” gauges. The “Field” is actually the longest: “The GO gauge corresponds to the SAAMI minimum chamber length, while the FIELD gauge usually matches the maximum chamber depth, or slightly less. NO-GO gauges are an intermediate length between minimum and maximum, that, technically, is a voluntary dimension. A firearm that closes on a NO-GO gauge and does not close on a FIELD gauge may not give good accuracy and may have very short cartridge case life from the ammunition re-loader’s standpoint.”
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Forum member Rich DeSimone uses a handy “Stub Gauge” for setting shoulder “bump” and seating depth. The gauge is made from a section of barrel lopped off when the muzzle is crowned. The chambering reamer is run in about 1/4 of the way, enough to capture the neck and shoulder area of the case. Rich then uses his full-length die to “bump” a master case with the ideal amount of headspace for easy feeding and extraction. He takes that case and sets it in this Stub Gauge, and measures from the front of the gauge to the rim. He can then quickly compare any fired case to a his “master” case with optimal headspace. Since the gauge measures off the shoulder datum, this tells him how much to bump his fired brass.
In addition, the Stub Gauge can be used to set bullet seating-depth. Rich has a channel cut transversely on one side of the gauge, exposing the throat area. Since the interior of the gauge is identical to the chamber in his gun, this lets him see where a seated bullet engages the rifling. He can tinker with bullet seating length until he gets just the right amount of land contact on the bullet, confirmed visually. Then he measures the case OAL and sets his seating dies accordingly. This is much handier than using a Hornady Tool to measure distance to the lands.
But what happens when the throat wears and moves out on your live barrel — making the actual length to lands different (slightly longer) than before. Well, the stub gauge is still valuable as a known starting point. As your barrel’s throat wears, you may seat your bullets out further to “chase the lands”, but the gauge provides a constant land engagement point, in the barrel’s “as new” condition. By measuring the difference between the land contact point on the gauge and the actual contact point on your barrel, you can determine throat “migration”.
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Some of our readers have questioned how to set up their body dies or full-length sizing dies. Specifically, AFTER sizing, they wonder how much resistance they should feel when closing their bolt.
Forum member Preacher explains:
“A little resistance is a good, when it’s time for a big hammer it’s bad…. Keep your full-length die set up to just bump the shoulder back when they get a little too tight going into the chamber, and you’ll be good to go.”
To quantify what Preacher says, for starters, we suggest setting your body die, or full-length sizing die, to have .0015″ of “bump”. NOTE: This assumes that your die is a good match to your chamber. If your sizing or body die is too big at the base you could push the shoulder back .003″ and still have “sticky case” syndrome. Also, the .0015″ spec is for bolt guns. For AR15s you need to bump the shoulder of your cases .003″ – .005″, for enhanced reliability. For those who have never worked with a body die, bump die, or Full-length sizing die, to increase bump, you loosen lock-ring and screw the die in further (move die down relative to shell-holder). A small amount (just a few degrees) of die rotation can make a difference. To reduce bump you screw the die out (move die up). Re-set lock-ring to match changes in die up/down position.
That .0015″ is a good starting point, but some shooters prefer to refine this by feel. Forum member Chuckhunter notes: “To get a better feel, remove the firing pin from your bolt. This will give you the actual feel of the case without the resistance of the firing pin spring. I always do this when setting up my FL dies by feel. I lock the die in when there is just the very slightest resistance on the bolt and I mean very slight.” Chino69 concurs: “Remove the firing pin to get the proper feel. With no brass in the chamber, the bolt handle should drop down into its recess from the full-open position. Now insert a piece of fire-formed brass with the primer removed. The bolt handle should go to the mid-closed position, requiring an assist to cam home. Do this several times to familiarize yourself with the feel. This is how you want your dies to size your brass, to achieve minimal headspace and a nearly glove-like fit in your chamber.”
We caution that, no matter how well you have developed a “feel” for bolt-closing resistance, once you’ve worked out your die setting, you should always measure the actual amount of shoulder bump to ensure that you are not pushing the shoulder too far back. This is an important safety check. You can measure this using a comparator that attaches to your caliper jaws, or alternatively, use a sized pistol case with the primer removed. See Poor Man’s Headspace Gauge.
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This illustration shows headspace measurement for the popular .308 Winchester cartridge, which headspaces on the shoulder. Image copyright 2015 Ultimate Reloader.
In this Brownells Tech Tip, Brownells gun tech Steve Ostrem explains what headspace is and why it’s one of the most critical measurements for nearly all firearms. Even if you’re an experienced rifle shooter, it’s worth watching this video to refresh your understanding of headspace measurements, and the correct use of “GO” and “NO-GO” gauges.
Headspace Definition
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct positioning. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56x45mm NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the forward rim of the cartridge.
If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed and the cartridge case may rupture, possibly damaging the firearm and injuring the shooter. (Source: Wikipedia)
Headspace Gauges
Headspace is measured with a set of two headspace gauges: a “Go” gauge, and a “No-Go” gauge. Headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a “Go” and a “No-Go” gauge are required for a gunsmith to headspace a firearm properly. A third gauge, the “Field” gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the “factory specs” measured by the “Go” and “No-Go” gauges. A bolt that closes on “No-Go” but not on “Field” is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec. (Source: Wikipedia)
To learn more, read Brownell’s longer article Headspace Gauges and How to Use Them. Among other things, this explains the relative lengths of “Go”, “No-Go”, and “Field” gauges. The “Field” is actually the longest: “The GO gauge corresponds to the SAAMI (Sporting Arms & Ammunition Manufacturer’s Institute) minimum chamber length, while the FIELD gauge usually matches the maximum chamber depth, or slightly less. NO-GO gauges are an intermediate length between minimum and maximum, that, technically, is a voluntary dimension. A firearm that closes on a NO-GO gauge and does not close on a FIELD gauge may not give good accuracy and may have very short cartridge case life from the ammunition re-loader’s standpoint.”
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The Tactical Rifle Shooters Facebook Group recently showcased tools used to measure case headspace before and after “bumping” the shoulder. After a case is fired, hand-loaders who full-length size their cases will typically bump the shoulders back anywhere from .001″ to .0035″, depending on the rifle and application. With our 6mmBR and Dasher cases we like about .0015″ bump.
You want the amount of case sizing and bump to be the same for all your brass. To ensure uniformity, it makes sense to measure your cases before and after the FL sizing process. When we have time, we check every case. Other folks will simply check the first 3-4 cases coming out of the FL sizing die to ensure the FL die setting is correct and delivering desired headspace/bump.
1. Whidden Gunworks Shoulder Bump Gauge
There are a variety of tools that can be used to measure shoulder bump. Our favorite is a special cartridge-specific bushing made by Whidden Gunworks. The Whidden Shoulder Bump Gauge enables you to adjust your sizing die to the desired measurement. The bump gauge is attached to your calipers with a set screw and determines the measurement from the base to the shoulder of the case. The photo below, from Tactical Rifle Shooters, shows the Whidden Bump Gauge for the .375 CheyTac cartridge.
2. Dave Manson Vertical Comparator with Dial Read-Out
Dave Manson states: “This tool was designed to make life easier for the advanced shooter and re-loader by allowing precise measurement of ammunition, case, and chamber headspace. With this information, the re-loader will be able to fine-tune clearances and fits between his ammunition and chamber, with resultant improvements in accuracy and case life.” The functions of the Comparator are:
1. Measure headspace of factory or reloaded ammunition
2. Quantify chamber headspace by measuring headspace of a fired case
3. Ensure minimal shoulder set-back when setting up re-loading dies
4. Compare base-to-ogive length to ensure consistent bullet-to-rifling relationship.
In addition to the Dial Indicator and Stand, the $130.00 Vertical Comparator is supplied with multiple Datum Blocks of precise length and inside diameter (.3300″/.3750″/.4000″/.4375″). MORE INFO HERE.
3. Hornady L-N-L Headspace Comparator System
Hornady makes comparator gauges matched to the red comparator holder that mounts on your caliper. These Lock-N-Load Headspace Gauges are inexpensive. You can get a set of five gauges for $31.99. Hornady explains: “The Lock-N-Load® Headspace Comparator… gauge measures variations in brass before and after firing or re-sizing. It allows for headspace comparison between fire-formed brass and re-sized brass.” IMPORTANT: Hornady states: “To determine the proper bushing diameter for your cartridge, simply add the neck diameter and the shoulder diameter and divide that number by two. Use the bushing closest to that number.” Hornady offers five: .330″, .350″, .375″, .400″, and .420″.
One tip — We have found the Hornady gauges may vary a little from unit to unit even with the same nominal size. If you have more than one gauge for the same cartridge, test each on your brass — you may then note a slight difference in your bump measurements.
4. Pistol Brass Case DIY Bump Gauge
Last is a “field expedient” set-up if you do not have any of the comparator tools shown above. A sized .45 ACP case (or other suitable pistol case) can be used to measure shoulder bump. The mouth of the pistol case sits on the shoulder of your rifle cartridge brass.
Make sure the .45 ACP case is trimmed square and that it is round. We recommend you first run it through an expander, then size it, trim it and chamfer. Next, take the .45 ACP case and slip it over the neck of a fired, unsized rifle case with the primer removed. Align the two cases between the jaws of your calipers and note the length from rim to rim (See left photo below).
OK, now you have the length for a fired rifle case BEFORE sizing. Next, take a full-length sized rifle case (without primer) and do the same thing, placing the .45 ACP case over the neck of the FL-sized case (Right Photo). The difference between the two numbers is the amount of “bump” or set-back you are applying to the shoulder. Here the difference is .0015″. The amount of bump you need varies with your chamber and your load, but .0015-.002″ is a good initial setting.
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Do you know what the inside of a rifle chamber (and throat zone) really looks like? Do you understand the concept of headspace and why it’s important? If not, you should read the Brownells GunTech article Gauging Success – Minimum Headspace and Maximum COL. This article explains the basics of headspace and shows how to measure headspace (and length to lands) in your barrels with precision. The article also explains how to adjust your full-length sizing dies to “bump the shoulder” as needed.
Why is headspace important? The article explains: “Controlling headspace and setting proper C.O.L. also represent improved safety and reduced cost of handloading. Excessive headspace can cause case head separation and gases in excess of 60,000 PSI escaping from a rifle’s chamber. Too little headspace can result in a chamber forced bullet crimp and a bullet that becomes an obstruction more than a properly secured projectile. Excessive C.O.L. can result in a rifling-bound bullet, a condition that could result in spikes of excessive pressure.” [Editor’s NOTE: It is common for competitive benchrest shooters to seat bullets into the rifling. This can be done safely if you reduce your loads accordingly. With some bullets we often see best accuracy .010″ (or more) into the lands. However, this can generate more pressure than the same bullet seated .010″ away from initial lands contact. As with all reloading, start low and work up gradually.]
How is headspace specified? Most cartridges used within the United States are defined within ANSI/SAAMI Z299.3-4. Brownells explains: “In the case of the .243 Winchester, as an example, there are pressure specifications, cartridge drawings and, as pictured above, chamber drawings. Armed with a chamber drawing, each manufacturer producing a firearm for the .243 Winchester knows the proper standard dimension to cut chambers and set headspace. Notice there are two headspace reference dimensions for the chamber. The upper is a place in the chamber where the shoulder is .400″ in diameter; the “datum” or “basic” line. The lower is the 1.630″~1.640″ minimum – maximum dimension from the breech face (bolt face) to that point in the chamber that measures .400″.”
The actual headspace of any firearm is the distance from the breech face to the point in the chamber that is intended to prevent forward motion of a cartridge.
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This barrel’s shoulder was 0.025″ off the action because Red Locktite had been used on the threads.
Gunsmith Thomas ‘Speedy’ Gonzales offered this interesting report about how NOT to headspace a barrel. Hopefully you never discover something like this…
“A good friend and customer sent this rig in for repair after FedEx damaged the rifle during inbound transport from another smith. After repairing the stock and rebedding it, I decided to re-polish the barrel to make the repair perfect. Well this just added insult to injury as the barrel did not want to come off. After a few choice words, the barrel finally broke free only to reveal something very disturbing. It seems the barrel had been ‘headspaced’ by using RED Loctite to hold it in place.” [Editor: That’s definitely NOT how barrels should be fitted.]
Speedy was not happy: “I hope the smith that did this sees the photos and realized what jeopardy he put my customer in or anyone who shot the rifle for that matter. When cleaned up, the shoulder on the barrel was over 0.0250″ (25 thousandths) away from the face of the receiver.” [Editor: That’s a lot in this business]. Check out the images below to see how much the barrel rotated further inward when cleaned up. The barrel spun in nearly another eighth-turn or more. Not good.
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Can you list all the serious problems that excessive headspace can cause? For that matter, could you even explain what, exactly, is meant by the term “headspace”? If not, you should watch this instructional video from Brownells. This video defines the term “headspace”, explains why proper headspacing is critically important, and illustrates how headspace gauges work.
Headspace is a measurement from the bolt-face to a point in the chamber. This point of measurement will vary based on the type of cartridge. Improper headspace, either excessive or (conversely) under SAAMI specifications, can cause a variety of problems, many serious.
Problems Caused by Too Much Headspace
Excessive headspace issues can include: light primer strikes, failure to fire, bulged/blown cases, case separations, split shoulders, or unseated primers after firing. Case ruptures caused by excessive headspace can lead to catastrophic failures causing serious injury. That is why headspace is such an important measurement.
Problems Cause by Too Little Headspace
Insufficent (or excessively tight) headspace can prevent the firearm from going into battery, resulting in failure to fire or deformation of the cartridge case. Various feeding and functioning problems can be caused by cases with too little headspace, even if a round can be chambered (with effort).
Share the post "Headspace 101: What You Need to Know"
This illustration shows headspace measurement for the popular .308 Winchester cartridge, which headspaces on the shoulder. Image copyright 2015 Ultimate Reloader.
In this Brownells Tech Tip, Brownells gun tech Steve Ostrem explains what headspace is and why it’s one of the most critical measurements for nearly all firearms. Even if you’re an experienced rifle shooter, it’s worth watching this video to refresh your understanding of headspace measurements, and the correct use of “GO” and “NO-GO” gauges.
Headspace Definition
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct positioning. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56x45mm NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the forward rim of the cartridge.
If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed and the cartridge case may rupture, possibly damaging the firearm and injuring the shooter. (Source: Wikipedia)
Headspace Gauges
Headspace is measured with a set of two headspace gauges: a “Go” gauge, and a “No-Go” gauge. Headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a “Go” and a “No-Go” gauge are required for a gunsmith to headspace a firearm properly. A third gauge, the “Field” gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the “factory specs” measured by the “Go” and “No-Go” gauges. A bolt that closes on “No-Go” but not on “Field” is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec. (Source: Wikipedia)
To learn more, read Brownell’s longer article Headspace Gauges and How to Use Them. Among other things, this explains the relative lengths of “Go”, “No-Go”, and “Field” gauges. The “Field” is actually the longest: “The GO gauge corresponds to the SAAMI (Sporting Arms & Ammunition Manufacturer’s Institute) minimum chamber length, while the FIELD gauge usually matches the maximum chamber depth, or slightly less. NO-GO gauges are an intermediate length between minimum and maximum, that, technically, is a voluntary dimension. A firearm that closes on a NO-GO gauge and does not close on a FIELD gauge may not give good accuracy and may have very short cartridge case life from the ammunition re-loader’s standpoint.”
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Headspace Gauges
Headspace Gauges
Forum member Rich DeSimone uses a handy “Stub Gauge” for setting shoulder “bump” and seating depth. The gauge is made from a section of barrel lopped off when the muzzle is crowned. The chambering reamer is run in about 1/4 of the way, enough to capture the neck and shoulder area of the case. Rich then uses his full-length die to “bump” a master case with the ideal amount of headspace for easy feeding and extraction. He takes that case and sets it in this Stub Gauge, and measures from the front of the gauge to the rim. He can then quickly compare any fired case to a his “master” case with optimal headspace. Since the gauge measures off the shoulder datum, this tells him how much to bump his fired brass.
To quantify what Preacher says, for starters, we suggest setting your body die, or full-length sizing die, to have .0015″ of “bump”. NOTE: This assumes that your die is a good match to your chamber. If your sizing or body die is too big at the base you could push the shoulder back .003″ and still have “sticky case” syndrome. Also, the .0015″ spec is for bolt guns. For AR15s you need to bump the shoulder of your cases .003″ – .005″, for enhanced reliability. For those who have never worked with a body die, bump die, or Full-length sizing die, to increase bump, you loosen lock-ring and screw the die in further (move die down relative to shell-holder). A small amount (just a few degrees) of die rotation can make a difference. To reduce bump you screw the die out (move die up). Re-set lock-ring to match changes in die up/down position.
