To Gauge or Not to Gauge?

By Fred Zeglin
AGI Instructor, Author,
Lassen College Gunsmithing Graduate and Cartridge Designer.

Liability is the question

I talk to hundreds of gunsmiths each month. It surprises me how often these “professionals” decide not to use a headspace gauge when they are available to them. They say, “No, that’s OK, I’ll just use the brass.”


The simple headspace gauge. So small, so inexpensive, and so valuable.

The simple headspace gauge. So small, so inexpensive, and so valuable.

The reason we are still using brass cases in firearms after more than a century is that so far brass still provides the best mechanism for safely sealing the breach during the firing of the round. Brass is surprisingly forgiving and acts a bit like an inflatable gasket when pressures rise. The properties of brass have saved many an experimenter who did not understand the problems of headspace and pressure.

Now I will be the first to admit that, on some rare occasions, you may not have a choice with regard to gauges. Obviously, if the gauge is not available and delivery time for one from a reamer maker or a rental agency is too long, you may be forced to move forward. Using brass or ammo as the gauge to set headspace is expedient, not ideal. Nobody likes a client to be angry about delays, so we push ahead with an expedient method of headspacing . . . to our peril.

Before you decide to send me a nasty-gram and defend the practice as acceptable, let’s talk about it for a bit. The primary topic of this discussion revolves around the fact that brass’s dimensions can and do vary widely due to manufacturing tolerances.

There are instances where the practice is not really a danger or a liability. However, that does not mean it’s a good idea all the time. Starting with a list of the times it might be safe and reasonable to do so.

1. Rimmed cases

2. Low pressure cases

3. Wildcat designs

With rimmed cases you’re pretty safe for two reasons. First, most are what we consider low pressure or are straight walled so pressure drops dramatically for every increment the bullet moves down the barrel. Second, single shots and lever guns that normally accept these cartridges will function fine with a small amount of excess headspace.

If we are talking about revolvers, using brass for headspace is something to be cautious about. It can certainly be done; however, I would check the rim thickness of my cases against the SAAMI specs for the cartridge at hand. If you have a minimum spec piece of brass and you set the gun to minimum headspace with it, then it will not be long before the client complains that the gun jams up and the cylinder will not turn.

If your client is a reloader or shoots commercial reloads the problem might be intermittent. Why is that? Because his brass is probably mixed from lots and makers so there is no rhyme or reason as to when the problem appears.

When we talk about low pressure cartridges we are referring to cartridges that operate under 40,000 PSI in rifles. Black Powder cartridges are good examples of this type of cartridge. The most popular of all is probably the .30-30 Winchester. Why is headspace less important with these cases? Because they will generally work safely with a large amount of excess headspace, simply because the brass is strong enough to deal with the low pressures generated. Therefore your liability is minimal.

In the example of the .30-30, you can actually have so much headspace in a 94 Winchester that the cartridge will sometimes misfire and yet you will never have a case head separation. That is because this cartridge, when loaded to factory levels, cannot overcome the strength of the brass to produce a catastrophic failure.

Gauging the chamber when a gun comes in, and your new chamber when the gun goes out, demonstrates professionalism to the client and reduces your legal liability.

Gauging the chamber when a gun comes in, and your new chamber when the gun goes out, demonstrates professionalism to the client and reduces your legal liability.

When you jump up in pressure though, as with the .375 Winchester vs. a .38-55 you will see that catastrophic failure is a real concern. Why? Because there is much more pressure in the .375. Thus, using brass to headspace a midrange cartridge (40,000 PSI to 47,000 PSI) is a more delicate decision. Again you would want to be sure about the rim thickness of your sample brass vs. the SAAMI specifications for that cartridge.

With wildcat designs of your own making, headspace is totally up to you the maker. That is not to say it’s not important. You should set a standard for any design you come up with. The reason for doing this is so you can show that you work to specific dimensions if something were to go wrong. Plus it makes it much easier for you to diagnose problems if you know exactly what the dimensions are supposed to be.

If we are talking about an existing wildcat, then you should use the correct headspace gauge for that cartridge. If you fly by the seat of your pants you have no way of proving the gun was correct when it left your shop. Which brings up an important point, you should always test fire and retain at least one piece of brass for that gun. It makes your files a little fatter but it’s worth it when you need to diagnose a problem when a gun comes back or you have to show that things have changed since you last saw the gun.

A .257 Roberts Ackley Improved reamer, one of the more popular Ackley “Wildcat” chamberings. Case fired in improved chamber on left, case fired in standard chamber on right.

A .257 Roberts Ackley Improved reamer, one of the more popular Ackley “Wildcat” chamberings. Case fired in improved chamber on left, case fired in standard chamber on right.

One way to handle the fired cases is to simply put them in a bag with the invoice number and file them on an annual basis with your other records. A box for each year with nothing but your exemplar brass does not take up much room and is easy to locate when you need it. It’s habits like these that can save you time and money when working out a problem. Many times the client has changed the gun or damaged it in some way, or taken it to another gunsmith and these fired cases are a record of how the gun left your shop.

If there are times when using brass to gauge headspace is OK then there must be times when it’s a bad idea.

1. High intensity cartridges

2. Wildcat cartridges

3. Ackley Improved cartridges

High intensity cartridges run from 47,000 PSI to 65,000 PSI. A high intensity cartridge with a little headspace can cause all kinds of problems. Excess headspace will allow brass to stretch at the web just ahead of the solid case head. This will lead to case head separations in as few as one loading depending on the dimensions of the chamber in question, and the dimensions of the brass in question. An astute reloader will set his dies to minimize case stretch, but you cannot rely on the reloader to understand this issue. I do recommend sharing information with your client to educate them on proper loading techniques.

Only 6.7 thousandths of an inch between gauges for “Go”, bottom, and “No Go”, top.

Only 6.7 thousandths of an inch between gauges for “Go”, bottom, and “No Go”, top.

If you have a client who loves to ride the wild edge of velocity and pressure (and we all do!), keeping headspace to a minimum is an absolute necessity. When a cartridge is fired and excess headspace is present, or in other words, an excessive gap between the case head and the bolt face, the action will take a pounding with every shot. On top of that, a hot rodding reloader is pounding the gun even harder, in some cases approaching a “proof load” in terms of pressure with every shot. You’re not responsible for what they do, but you can still protect yourself from liability by keeping headspace to zero.

Here he goes talking out both sides of his mouth . . .

With wildcat cartridges using brass for headspace is a bad idea. As mentioned before brass can vary widely in quality and dimensions.

If the wildcat is of your own design, then make a gauge or have one made so you’re always working to known dimensions. If it’s an existing wildcat then use the proper gauge. Over the years too many smiths have just done whatever they want for headspace. The problem is pervasive enough that the reloading die manufacturers now ask for a chamber print when they sell wildcat dies; even for old time well-known wildcats.

This brings us to Ackley Improved cartridges. There are no simpler cartridges to headspace correctly and yet they are the most abused and trashed chambering jobs of all. Maybe because the system is so simple, people try to make it more difficult. Also because they are so popular they have more opportunity for error.

Rimmed Ackley cartridges use the gauges from the parent cartridge. No other gauge is needed. Why? Because these gauges work on the rim of the case, not the shoulder, so the shape of the body is almost immaterial. Belted cases also work this way.

Rimless, semi-rimless, and rebated cases for Ackley Improved designs all use an Ackley gauge that is .004″ shorter than the standard go gauge for caliber. The go gauge from the factory parent case becomes the no-go for the Ackley chamber. This insures that the factory cartridge can be fired in the chamber safely, thus fireforming to the “improved” shape.

An Ackley gauge being .004″ shorter in length causes the junction of the neck and shoulder to be the headspace point for the factory case in the Ackley chamber. This method means that the factory case is pinched or trapped between the bolt face and the chamber so tightly that it can then be fired safely. When fired, the brass balloons out to fit the new Ackley type chamber.

To further complicate the gauge situation for Ackley cartridges, some of the reamer makers now offer Ackley gauges with 40 degree shoulders, instead of the traditional design that had the original factory shoulder angle. The only thing this should change is the way the gauge interacts with the chamber. Otherwise the measurements should all work out the same. If you’re confused, a matched set of gauges for caliber will alleviate any concerns.

Standards and Tolerances . . .

The firearms industry utilizes voluntary standards. SAAMI is the library and registry for these standards. Utilize the data SAAMI provides, it is invaluable. SAAMI does not register standards for wildcats. Only members of SAAMI can submit cartridges for standardization. However, they will accept prints of your wildcats as a record and this could potentially save your bacon some day if you plan to market a cartridge actively.

The voluntary standards available from SAAMI protect us all from liability if we stay within them. The other option would be to have a government agency take over standards. If you think that sounds like a bad idea then you understand what prompted me to write this article. Gauges are your best defense against liability issues, so whenever possible; use them. You can download the standards from SAAMI at:

Measure carefully, ream properly, use the proper gauges, keep samples, keep records, and work like the professional you are.

Measure carefully, ream properly, use the proper gauges, keep samples, keep records, and work like the professional you are.

When you read the specifications for a particular cartridge you will note that there are tolerances called out for cartridge and chambers. Take the time to look them over, become aware of the fact there are acceptable standards already established. There is no reason for the gunsmith to fly by the seat of his pants. Even when working with a wildcat, many of the SAAMI standards for the parent case will still apply. Once you understand these tolerances you will be better able to diagnose problems with chambers and headspace much faster.

Now go forth and measure thy success.

Fred is a graduate of the Lassen College Gunsmithing program, a professional gunsmith and custom rifle builder for over 30 years, a former production manager for a major barrel making company, designer of the acclaimed Hawk series of cartridges, freelance author, wildcat cartridge designer, and AGI instructor.

3 Responses to To Gauge or Not to Gauge?

    • It’s the method Bob teaches in the Professional Gunsmithing course. If it’s no good, why does AGI teach it?

  1. I re barreled a small ring Mauser (96) from 7mm to 308, for a gauge to measure the head space, I used a resized brass, pored it full of lead and included steel in the fill. I have continued to use this gauge for about 30 years now. I would not use just a brass, brass will flex and give an unreliable test.