Headspace: Measurements and Corrections

A customer recently brought me a Springfield ’03-A3 for testing. He watched while I stripped the bolt, cleaned the chamber, and wiped the oil off my gauges. I demonstrated the bolt stopping on the no-go gauge, and he was quite surprised. He told me that he had taken it to a gun dealer who had simply stuffed a no-go gauge into the chamber and hammered the bolt shut with a plastic mallet. I was very surprised that this treatment hadn’t damaged the chamber.

After giving the customer a brief education in headspace, I don’t think he’ll be going back to that dealer again. In this report, I’ll cover the same areas, but in somewhat greater detail. Okay, I Need to Check Headspace First, let me restate the obvious: No force should ever be used when closing an action on a headspace gauge. None. Ever. And second’also obvious’headspace cannot be measured without the proper gauge or gauges.

As I stated in my previous article ("What Is Headspace?, March 2006), factory ammunition is loaded to specified tolerances to ensure it will fit in any properly sized firearm chamber. This means it can be 0.010 inch (or more) smaller or shorter than even a minimum chamber. Since you don’t know how much undersize a given factory cartridge is, it’s impossible therefore to use one to obtain anything resembling an accurate measurement of chamber headspace.

An undersized chamber can result in difficult extraction or extreme pressure transients, remember? If the chamber isn’t sufficiently large to allow the case to release the bullet, serious problems can occur. Trying to guess how much to add to a factory cartridge to get an accurate chamber measurement is like loading your own ammo using a soup spoon to measure the powder.

In my opinion, anyone who uses or advises the use of a cartridge or cartridge case as a headspace gauge is entirely ignorant of the magnitude of damage even a small error can cause. The next important item to know is that it’s nearly impossible to obtain any kind of reasonable measurement unless there is no outside force acting on the weapon’s action during testing.

This means that any part of the firearm that touches the gauge when the action is closed (other than the breechface and chamber, of course) must be removed, and any action springs must come out. This means that extractors and/or ejectors will generally need to be removed from those arms that have plunger-type ejectors and spring-loaded extractors. If you can’t disassemble the weapon’s bolt, you can’t accurately measure headspace. Take it to someone who can.

Bolts with internal plunger-type ejectors must have the ejectors removed before trying to measure headspace. Remove firing pins, striker assemblies, and other bolt internals. Semi-autos need to have their recoil springs removed before trying to check headspace. Anything touching the gauge while a measurement is being attempted will affect the accuracy of the reading. Anything that aids or opposes action closing will absolutely ruin any chance of precision measurement.

Since we are dealing with such tiny tolerances, everything must be done to ensure an accurate measurement. This means the chamber, bolt/ breechface, ratchet/ejector (in swingout revolvers), and headspace gauge must be clean and free of any rust or foreign material. Thoroughly scrub the chamber and bolt/breechface with suitable solvents. Remove any solvent residue and check all surfaces for cleanliness. Now, we’ll proceed depending upon the type of arm to be tested.

The Actual Checking Procedure Actions should always close completely (bolts locking into battery) with no resistance on ago gauge. Should you encounter resistance when closing an action on any headspace gauge, stop! Clean the chamber again, clean the gauge again, and examine everything carefully for dirt or burrs that may be interfering with the measurement. This is especially true of no-go and field measurements where you’re expecting to encounter this condition.

After all, you don’t want to miss along headspace condition any more than you want to "correct" a shortchamber condition only to find out you have actually caused another problem. Revolvers. Revolvers are checked in a couple of ways, depending on whether they have countersunk chambers

When checking headspace on a revolver, consult the manufacturer’s specs for proper tolerances; no-go measurements differ a few thousandths from make to make. This is a no-go gauge in a semi-automatic pistol. The slide is not in battery, indicating that the no-go gauge is preventing complete lockup. or not.

Those with countersunk chambers will require ago gauge to be checked properly, whereas those without countersunk chambers can be checked either with suitable gauges or by using feeler gauges to check cylinder-to-standing-breech clearance. My preferred method of testing revolvers works equally well with any revolver type. It is the easiest and most accurate method for testing revolvers with countersunk chambers and utilizes only the go gauge.

The chamber containing the gauge is rotated under the hammer and feeler gauges are used to check the remaining clearance ( Photo 1). Semi-Automatic Pistols. Semi-autos need to be checked for barrel lockup before gauging. With the slide in battery, take careful note (or a photograph) of exactly where the slide is With a no-go gauge in a bolt-action rifle, it’s okay for the bolt to begin to turn, but it should not close completely. This photo shows a no-go gauge in a Remington 740.

Note the mark on the locking lugs (1), the unmarked space showing the amount the bolt lacks to be fully in battery (2), and the gap between the bolt and bolt carrier (3) signifying that the bolt is not fully locked and headspace is okay. aligned with some point on the frame (or how far the barrel engages in the ejection port). Then remove the extractor and recoil assembly from the slide. The gauge is chambered and the slide is gently pushed forward until it stops.

The slide/barrel should return to the exact same position with ago gauge. The slide/barrel should stop somewhere short of fully in battery with a no-go ( Photo 2). Long arm testing is as varied as longgun action types. I’ll detail a few here, with the idea that you can extrapolate

these methods to whatever action type is on your bench. Break-Open Actions. The gauge is inserted into the chamber and the action is gently closed. The action should not latch shut on a no-go. Should the action close on a no-go gauge, it’s imperative that you determine the cause of the problem before messing with the chamber dimensions, as worn hinge surfaces are very common in these action types. Rotating-Bolt Actions.

The bolt may begin to close on a no-go gauge but should not rotate into the fully-locked position ( see Photo 3 ). On some actions (such as the Remington 74- and 76-type rifles), it may be helpful to mark the in-battery position of the bolt versus the bolt carrier or barrel extension so that a reference point is available for comparison. I use a permanent marker (or a grease pencil, also known as a "China marker") rather than scribing a mark.

Simply mark the bolt with the chamber empty, insert the gauge, then gently close the action and check for an unmarked area between the line and the bolt carrier or barrel extension. Any visi- Here’s a no-go gauge in an AR-type rotary-bolt semiauto rifle. The bolt carrier will not move up flush with the upper receiver, indicating that the bolt is not completely locked in battery and headspace is acceptable. The no-go gauge is shown here in another type of rotary-bolt semi-auto rifle.

The bolt is not completely locked into its locking lug recesses, indicating that headspace is okay. ble unmarked bolt steel between the line and the reference point indicates the headspace is less than no-go ( Photo 4). The bolt is not visible nor accessible in AR-type rifles, so you must observe the bolt carrier. The carrier should not move up flush with the upper receiver with a no-go in place (Photo 5). Exposed rotating-bolt actions are among the easiest to check.

This includes popular numbers like the US M1, M1A, and M1 carbine. The stripped bolt should not close fully on a no-go ( Photo 6). Tipping-Bolt Actions. The bolt may begin to cam into the locking recess, but it should not go all the way into battery. A useful method for viewing tolerances here is to color the bolt’s locking surface with a permanent marker or Prussian blue, then operate the bolt/carrier enough to establish a wear pattern. Then insert the gauge and observe the locking surface of the bolt.

If any shiny area is visible, the bolt is not fully locked into battery and thus headspace is less than no-go (Photo 7). What About the "Field" Gauge? Afield gauge represents the largest permissible chamber that can be safely employed with standard ammunition. The field gauge is usually used when checking military or ex-military rifles, as it is not unusual for military or ex-military rifles to have chambers near or slightly larger than no-go (even when new).

The most important function of a military arm is to fire on demand under adverse conditions, so they often have generous chambers to accommodate dirty, dented, or suspect ammunition. This doesn’t cause a problem in military use, because military ammo generally has thicker case heads than commercial ammo. Afield gauge is employed in the same fashion as a no-go.

If a rifle’s bolt will go fully into battery with afield gauge in the chamber, the rifle has very excessive headspace and should not be fired until the problem is corrected. Correcting Insufficient Headspace Conditions Revolvers with insufficient headspace can be corrected in a couple of ways, depending on whether the ratchet is part of the cylinder or a separate part. Of course, cylinder replacement is always one option, but is sometimes not feasible.

For revolvers where the ratchet is apart of the cylinder, the only option available is to turn a small amount off the back of the cylinder, leaving the ratchet untouched. With cylinders with separate ratchets you

In this photo of a no-go gauge in a tipping-bolt rifle (MAS 1949-56), note the shiny area indicating headspace is acceptable. (The bolt carrier is removed for clarity.) have the option of ratchet replacement or turning the back of the cylinder (with the ratchet/extractor assembly installed) to increase the headspace to acceptable clearances. Cylinders with countersunk chambers can sometimes be corrected by re-reaming the chambers or by ratchet (or cylinder) replacement.

Semi-auto pistols will need their chamber reamed to correct dimensions or the barrel changed. Most long guns can have headspace increased by simply reaming the chamber to the proper size. Savage centerfire rifles can be adjusted as described in the next section. Correcting Excessive Headspace Conditions Should you be dealing with a poorly made firearm, the best advice I have is to tell the owner to retire the thing and replace it with something more durable.

Trying to make a silk purse from a sow’s ear is the surest road I know to civil and possibly criminal liability. Excessive headspace is a common problem with these firearms. Sometimes it’s due to wear and tear, sometimes to mistreatment, and sometimes to poor workmanship or improper materials used in construction. axis. Shims are available from Brownells and Midway USA, and a swaging tool (basically a tubing cutter with a dulled wheel) can be bought from Brownells or made by the enterprising gunsmith.

Semi-auto pistols will require barrel replacement. Shotgun headspace problems due to chamber conditions (not related to worn locking surfaces or other moving parts) can best be corrected by using tooling and rim inserts by Clymer (available from Brownells). Rifle headspace problems are corrected depending on the type of breeching system used. Most types of front-locking actions (such as Mauser-type bolt-actions, Remington boltactions, etc.) will require barrel setback or replacement.

It is possible to correct headspace in some rifles by changing bolts, but this is usually a hit-or-miss proposition. (Exception: SMLE rifles had several different sizes of bolt heads manufactured in order to allow headspace adjustment over the expected rifle life.) I’ve also seen rifles that have had minor corrections made by hard-chroming the bolt face to build it up, but this can only compensate for a few thousandths of error.

Savage centerfire rifles using a locking nut to retain the barrel are the easiest on which to set headspace. Lock the receiver in a receiver wrench (a Remington 700 receiver wrench will fit) and clamp the wrench in your bench vise. Loosen the lock nut with a Savage wrench and back the barrel Long headspace in a revolver that is not due to frame stretching often shows up as cylinder endshake.

This can be corrected by adjusting the cylinder’s longitudinal position on its axis by the use of shims, by swaging a groove on the axis, or by replacing the off a turn or so. Remove the extractor, ejector, and firing mechanism from the bolt. Then chamber ago gauge, and shut the bolt fully. Carefully screw the barrel in until it touches the go gauge (it will stop turning).

Back the barrel off just a fraction and tighten the lock nut snug. (It doesn’t need to be torqued until it squeaks.) The ideal position is where you can "feel" the gauge in the chamber as the bolt reaches the fully locked position, but absolutely no force is necessary to close the bolt. It takes a bit of experimentation, but is fairly easy to master. Rifles that use tipping-bolt rearlocking mechanisms usually have their headspace adjusted by the expedient of changing the bolt’s locking shoulder.

This is a "crossbar" in the receiver that the rear of the bolt cams down in front of. Examples include the SKS, FN49, FAL, SVT-38/40, MAS 1949 and 49-56, Ljungman, and Hakim. If a replacement shoulder is not available, it’s possible to weld up the existing unit and refit it. All applicable warnings about knowing your metallurgy and heat-treating methods apply here; if you leave the shoulder too soft it will wear out quickly, and if you leave it too hard it may shatter.

Rear-locking tipping-bolt rifles can also be modified by changing or refitting their locking shoulder. Care needs to be taken when addressing these rifles. That’s because it’s possible to set headspace properly while simultaneously having a dangerous amount of cartridge head exposure because the chamber is physically too shallow.

It’s important on these actions to ensure that the cartridge enters the chamber far enough that the front edge of its extraction groove is almost flush with the end of the chamber to guarantee sufficient support of the case head and web. Living with Excessive Headspace: It’s Possible (Sometimes) Revolvers with long headspace will sometimes misfire due to insufficient primer pellet crush when struck by the firing pin. It’s a major mistake to (continued on page 15)

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try to correct this by "improving" firing-pin protrusion through the frame, as this can result in pierced primers during cartridge setback when fired. Fix it or replace the revolver. Semi-auto pistols chambered for straight-walled cartridges can usually live with somewhat long chambers. Unless it’s a target pistol, it’s likely that no one will ever notice an excessive headspace condition on most of these pistols.

An extreme case of this condition would be when someone decides it’s a good idea to try shooting some .40 S&W ammo through a 10mm pistol. I’ve seen it done more than once, and the pistols all worked fine. According to SAAMI specs, the maximum length of a 10mm chamber is 1.004 inch, with the minimum length of a .40 S&W case being 0.840. This represents a theoretical 0.164inch excessive headspace problem!

However, as long as the round in the chamber is held in place by the pistol’s extractor, it fires and sometimes shoots reasonably well. Is this dumb? Of course; it’s never smart to intentionally chamber and fire ammo other than what the arm was manufactured to use. American Gunsmithing Association News Don’t forget to check out the center spread in this issue. Enroll in the AGI’s Complete DVD Gunsmithing Course and get an instant rebate of $200.

As a professional gunsmith, there’s never been a better time to expand your business, sharpen your skills, and broaden your knowledge. And if you can’t decide right now, you can always send for the introductory enrollment kit and Shotguns with long headspace will usually misfire and little can be done to live with the condition. I recommend fixing it or retiring the shotgun. Rifles in military calibers with headspace between no-go and field can usually be safely used with military brass.

If you or your customer reload, be advised that switching from commercial to military brass will entail charge reductions of between 5 and 15 percent, because the thicker military case head reduces case capacity. My advice is to reduce charge levels to starting loads and work your way back up carefully. I have seen many folks keep a rifle with excessive headspace in use by adjusting their reloading dies to prevent shoulder setback during resizing, thus "fitting" the case to the chamber.

This may seem acceptable on the surface, but it’s actually a very bad idea because the excessive headspace is causing abnormal case exposure in the web area in front of the head and can result in a blown case. Should this happen, you won’t like it. I’m thankful that I didn’t have one of these blowouts many years ago before I decided to gauge my Koreanera M1. Not only would the bolt close on afield gauge, the gauge would sample one-hour lesson for just $9.95.

Refer to the center spread in this issue for more details. Also available are three-ring binders that hold a year’s worth of American Gunsmith issues. Order several to house your collection safely and securely. Binders are $8.95 each (shipping and handling can Gunsmithing Reader Services, Beachwood, OH ks. Or call toll-free to order. rattle in the chamber. I found a replacement barrel and fixed it. Conclusions There are a few weapons to which these rules don’t apply.

These are mainly the delayed-blowback rollerlocked designs associated with H&K. This also includes the CETME rifles and those pistols using this system, such as the CZ-52. These actions require checking a gap between the bolt carrier and the bolt head when the action is in battery. They require no disassembly or cartridge headspace gauges, just an appropriately sized feeler gauge to measure the gap. When this gap falls below manufacturer’s specifications, it means that headspace has grown too long.

It’s possible to change bolt heads and/or rollers to adjust headspace in these arms’up to a point. However, these barrels will wear out just like any other. I’m thinking about an article on making your own gauges for those cartridges that are without commercially available units. This is not a job for the beginner, as it will require exacting measurements and top-flight measuring tools. It’s not impossible, however, and there are a few ways of "cheating" with existing gauges. I AGA Corner