September 23, 2010
Checking headspace on a revolver is easy and requires very little special equipment. If you are using a set of headspace gauges, each gauge is placed in a chamber, and the cylinder is closed to verify that it is within prescribed tolerances.
When you have a gunsmith shop, there are some jobs that folks will ask for on a pretty regular basis. You'll mount scopes, install recoil pads, glass-bed actions, and replace broken parts. These jobs keep you busy and help pay the rent. One of the more frequently requested jobs is to check headspace on a rifle. It seems most gun owners are aware of the importance of headspace and want to make sure their rifles are headspaced properly.
Oddly enough, I seldom have had a customer ask me to check headspace on a revolver. Sure, I often do this as part of other jobs, such as safety checks or diagnosing other problems in a wheelgun, but I probably have had no more than a dozen or so customers ask to check the headspace on their revolvers.
Headspace on a revolver can be just as important as on a centerfire rifle even though you seldom encounter situations where the negative consequences are as grave. Seriously excessive headspace in a high-power rifle can lead to a ruptured cartridge case and the release of hot gases under tremendous pressure into the action. If the gas escapes back into the shooter's face, the results can be disastrous.
Let's take a look at revolver headspace and how to check it. It's a bit different than with most rifles, but if you're a revolver shooter, you might want to know more about this. You never can tell when you'll want to check out a revolver, and yes, you can check headspace without using headspace gauges, but there are some things you need to know to do it right.
Headspace in a standard double-action centerfire revolver that fires a rimmed cartridge, such as the .38 Special, is measured from the point on the cylinder that supports the cartridge rim to the vertical face of the frame, or breechface, directly behind the cylinder. The gap between these two points must be large enough to permit the cartridge case to fully seat in the cylinder chamber yet small enough to support the head of the case when the cartridge is fired. If there's excessive headspace between the rear of the cartridge case and the face of the frame, the cartridge will not be supported. Upon firing, the cartridge case will move back out of the chamber until it is stopped by the frame. That's well and good, but the side of the cartridge case immediately ahead of the rim is no longer supported within the chamber. Depending upon the pressure of the load, the unsupported case could bulge or even rupture. This could damage the gun and possibly injure the shooter.
If the gap between the cylinder and the breechface is too small, then it may be impossible for the cylinder to lock into the frame or rotate properly when the gun is loaded.
For many years, the firearms industry group known as the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) set standards for critical dimensions for firearms and ammunition. According to SAAMI, the minimum acceptable distance, or headspace, between the rim seat on the cylinder and the breechface for a .38 Special revolver is .060 inch. The maximum acceptable headspace is .074 inch. Thus we have a tolerance of .014 inch between minimum and maximum headspace. As long as a revolver meets these standards, it is considered to have proper headspace. Gauges are produced to measure these critical dimensions. The gauge equivalent to minimum headspace is normally the "Go" gauge, while the gauge equal to the maximum headspace is often, but not always, the "No-Go" gauge. More on that later.
The breechface around the firing pin on this old Smith & Wesson .38-44 Heavy Duty has been peened out from dry-firing. This must be corrected in order to check headspace.
What about the ammo? According to SAAMI, the rim thickness of a standard .38 Special cartridge should be anywhere from .048 to .059 inch thick. By the way, even with the thickest rim at .059 inch, a cartridge would still be useable in a revolver with the minimum acceptable headspace of .060 inch.
Now that we have our specs, we can begin the process of checking headspace. The first step is to check the breechface of the revolver, with special attention to the area around the firing-pin hole. On many older revolvers that have seen a lot of use or dry-firing, the edge of the firing-pin hole in the breechface may be pushed up. This is caused by repeated hits from the firing pin as it passes through the breechface. This displaced metal around the firing-pin hole can catch the cartridge case and make cylinder rotation difficult, if not impossible. It can also make it impossible to get a true headspace measurement as you will measure from the top of this displaced metal rather than from the breechface. If the area around the firing-pin hole is not smooth, it should be carefully stoned and polished until it is nice and level.
Next, check the extractor. Remember that headspace is measured from the rim seat around the chamber to the breechface. On double-action revolvers, you generally have a separate extractor located in a machined recess in the rear of the cylinder. The rim of the cartridge rests partly on the extractor and partly on the rear face of the cylinder. Before measuring headspace, make sure the extractor is fully seated in the cylinder. Carbon, gunk, and powder residue can build up under the extractor and prevent it from fully seating. Also, on revolvers where there is a countersunk rim recess in the cylinder and extractor, make sure all the rim cuts in the extractor are even, clean, and to the same depth.
If there is any back and forth movement of the cylinder due to wear, be sure to always push the cylinder forward when actually checking headspace.
Checking Headspace With Precision Gauges
My preference when checking headspace is to use precision hardened and ground gauges. These are available from a number of different manufacturers.
My gauges were produced for the .357 Magnum but can be used in the .38 as well. One of the things you should always do is check your gauges, and on a rimmed gauge, that's really easy. My "Go" gauge has a rim thickness of .060 inch, and the "No-Go" gauge measures .0695 inch.
The fact that the "No-Go" gauge is smaller than the SAAMI maximum spec for headspace is not unusual. Many manufacturers and gunsmiths set the spec for their "No-Go" gauges at about
.010 inch above the minimum headspace for rimmed revolvers. In some instances, a revolver could accept the "No-Go" gauge and still be within the SAAMI acceptable limits.
Begin by placing the "Go" gauge in one of the chambers and carefully close the cylinder. Hold the hammer back just enough to allow the cylinder to rotate and turn the cylinder with your other hand. The cylinder should rotate easily with no drag on the gauge. Repeat the process, checking each chamber.
If you use a fired cartridge case to check headspace, you must first measure the rim thickness.
Next, place the "No-Go" gauge in one of the chambers. Carefully close the cylinder, draw the hammer back just a bit, and attempt to rotate the cylinder. More often than not the gauge will prevent the rotation of the cylinder. Don't try to force it! Repeat the process with each chamber. Using these gauges, we would now know that the headspace is between .060 and .0695 inch, which is well within industry standards.
Checking Headspace Without Gauges
If you don't have gauges, you can still check headspace using a fired or empty cartridge case. Don't even think of doing this with a live round!
After preparing the revolver, take the case you have selected and measure the thickness of the rim. Make a note of this figure. Place the empty cartridge case in a chamber and then rotate the cylinder until the case is positioned under the firing pin. Now take a feeler gauge and measure the distance between the rear of the head of the case and the breechface. Write that down and add that figure to the number you have for the thickness of the rim. That figure is your headspace for that one chamber. You need to repeat the process for each individual chamber and, of course, be sure to use the same empty case.
If you have misfires, light primer strikes, or bulged cases, excessive headspace might be the culprit. Using headspace gauges or a fired cartridge case and a feeler gauge, you can easily check this out.
Until next time, good luck and good gunsmithing!