The AR-15 is designed to be modular. This means you should be able to take the barrel off of one and put it on another without having to do much physical manipulation of the materials. In fact, this was one of the genuine innovations of the platform.

In a perfect world, this would be the case. The world is not perfect, and neither are AR-15 parts.

With that caveat in mind, let’s dive in deep to a measurement that matters: headspace.

A diagram of headspace orientation.
A diagram of headspace orientation.

What is headspace?

Headspace is the amount of room needed inside the chamber to hold a round secure while the gun is in battery. When the bolt closes (assuming it will close), there should be just enough room inside for the cartridge to be properly seated. Too much room or too little room will negatively effect performance and can be dangerous.

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AR-15 Headspace Explained

Engineers measure headspace as the distance between the face of the bolt and the datum line—a set point on the shoulder of a cartridge (this obviously varies from caliber to caliber). There’s no need for most of us to know what this measurement is, exactly—it is built into the design of the gun.

Headspace, in its optimal sense, allows for a bit of wiggle-room. Chambers are rarely clean and some guns have coatings that can add to their physical dimensions. As such, optimal headspace still allows for some small—though very small—size deviations.

How much slop is tolerated? We’re talking thousandths of an inch. Six thousandths. And it varies depending on .223 or 5.56, and by what type of gauge you’re using.

Every .223 round should be able to fit inside every .223 chamber. And the industry as a whole accepts .006” of deviation to ensure that they do.

There are gauges for every caliber, as this group from Clymer Tools shows.
There are gauges for every caliber, as this group from Clymer Tools shows.

AR -15 headspace check

One of the most common ways to measure headspace is with a go/no-go gauge. These are milled to the largest allowable cartridge size. If the bolt closes up on the go gauge, the headspace should be good, or at least there’s enough.

If the no-go gauge fits, though, this is bad. There’s too much room. While this isn’t unsafe, exactly, the extra room inside the chamber will be filled with the expanding brass case and it will maul your cases.

Most of us don’t care what happens to our brass, but reloaders do. Stretching and reforming, then stretching again… it is hell on the spent shells. With ammo shortages, we should all care about our brass. Even though you may not be reloading, odds are someone picking up the brass at your range is.

More advanced gauges

Slightly more advanced are a set of gauges designed for more accurate reads. If the no-go gauge says no-go, then these gauges come into play. Each is .001 larger than the last and their use should spell out just how far out of spec a chamber might be.

As we’re getting more and more reliant on electronics, a new gauge has been created that combines a cartridge shaped insert with a digital micrometer. These can break down the problem with even more detail.

Do I have to measure headspace?

This is one of those frequently asked questions. Don’t let the nuance of this escape you. Reloading matters to some of us, but not all of us, and especially not to the military. Some AR-15 armorers come from this school of thought and will often rely on field gauges to check headspace on guns. These field gauges are only meant to check for enough space for the gun to run reliably.

The military field gauge is truly a tool of the field and designed to spot-check guns that are already in service—not new guns that should have tighter tolerances.

If you are changing barrels or building from scratch, it is good to check your specs.
If you are changing barrels or building from scratch, it is good to check your specs.

Here’s the last thing to consider about measurements. There’s a ton of variation out there in both parts and gauges. It would be nice if we could all agree on a very specific set of measurements down to the thousandth of an inch, but our manufacturing capabilities (tool wear included) won’t allow for it in practical application.

As such, there’s not a perfect solution to this dilemma for most of us—those building ARs from parts. We work with what we have.

How to check headspace on an AR-15

Let’s dive in to some practical applications. Begin by safely field stripping the gun and cleaning everything well. Then take apart the bolt.

There are useful tools for these jobs. AR specific tools help. And tools meant for cleaning the bolt are really useful, too.

Take down the bolt. Remove the extractor. Remove the ejector.

Some folks want to put in gauges and cycle the action normally. Don’t. Much of the work will be done by hand and not with much force.

Place a go gauge in the chamber and slide the bolt into the barrel extension behind the gauge. As it seats, you should be able to rotate the bolt ahead of the locking lugs. If the bolt turns with minimal pressure, you have—at least—enough room for reliable operation.

If you don’t, or if you can’t turn the bolt, the chamber is short. Unless you’re a qualified AR armorer (and you wouldn’t have read this far, I doubt, if you were), it is time to take this to a professional and find out which of the parts is out of spec. As is, the rifle isn’t safe to use.

Let’s assume, as is most often the case, that the go gauge fit fine. Now repeat the process with the no-go gauge. In this case, the bolt should not rotate. If the bolt rotates on both the go and no-go gauges, then there’s too much room in the chamber.

Magpul PMAG 30 Round M2 MOE Mag - .223 / 5.56 NATO - MAG571
If you do any reloading, headspace will matter. If not, a bit of play isn’t going to sideline your gun.

Too involved?

Confused yet? This isn’t basic AR-building 101. There’s a technique that has to be practiced.

And maybe that’s the key. Some of my mentors—the men I always turn to to make sure I’m doing these things right—argue that you should test out gauges and skills on guns that you know are headspaced correctly. That way you’ll have a working understanding of what a brand-new rifle should read and how a well-worn rifle reads, too, before you start making physical changes to your guns.

How to headspace an AR-15

Don’t hesitate to turn to the manufacturer. If, for example, you buy a complete rifle from a manufacturer and something’s not right—send it back. People make guns and people make mistakes.

Watch out for used guns, DIY builds, and the gun-show-bargain. This is where you’ll end up buying a pig in a poke. Almost all of the new guns and reputable parts makers are running CNC machines, and—not insignificantly—their business models depend on parts working right, the first time, out of the box, off the shelf….

Two things to keep in mind, though. If you intend to fix (or have fixed) a headspace issue, it can only really go one way. If the chamber is too small, you can remove material.

If there’s too much headspace, you’re out of luck. There may be no safety issue, though, so the gun might appeal to that friend of yours who could care less about reloading, or the friend who only shoots steel-cased ammo.

If you do want to have the barrel reamed, it is again time to consult a professional.

Can you check AR-15 headspace without gauges?

Yes, you can. But understand that what you will learn will be of limited value if you are hoping to get headspace exact.

Safely remove the BCG. Pull the actual bolt head from the BCG and remove the extractor and ejector. Now put it all back together.

Insert a clean brass case into the chamber and reinsert the BCG into the upper and close the action. If it closes tight, you know you have enough (and maybe too much) headspace.

If the BCG won’t slide into battery on that case–which you’ll see because it sticks out the back of the upper (maybe as much as 1/8th of an inch), you don’t have enough headspace.

JP Enterprises Case Gauge Cut - Semi Auto - .223 Wylde
If you need to check your brass, this is a handy tool from JP Enterprises.

In some instances, a case will rattle just a bit in that closed chamber if there’s too much headspace. But it may not.

The real indicator will be the state of the brass after you fire a round. If you have more headspace than you need, the accuracy of the gun may suffer, but the brass may have odd areas of expansion that would (or should) prevent it from being reloaded.

It doesn’t hurt to have a set of go/no-go gauges. They make everything easier.

In the end

Headspace shouldn’t be a huge problem. Discussing it with such sincerity is kind of like talking about shark attacks in the ocean. Yes a few people get bitten every year, but the numbers are really small.

Just understand that the AR was designed to have some pretty flexible tolerances. These guns have to run in some absurd conditions. This isn’t optimal for reloaders, but reloading .223 or 5.56 is, at best, an afterthought.

For 99.99% of us, there’s no real issue with not enough headspace. It is so rare. And this means that the worst we could face is a bit too much.

And while hardly optimal, too much isn’t a safety issue.

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

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