Editor’s Note: This article is an excerpt from Gun smithing the AR-15: The Bench Manual by Patrick Sweeney.
Learn how to check headspace and adjust headspace on your AR-15 with this guide from master gunsmith Patrick Sweeney.
What is headspace? In simple terms, it is the amount of room in the chamber for the cartridge to rest before it is discharged. The measurement is defined as the distance from the face of the bolt to what is called the “datum line,” which is a particular location on the shoulder of the chamber of a given diameter, and a stated distance from the face of the bolt. The idea is to provide enough room for the loaded round to fit, even when the rifle is dirty, choked with carbon or just not squeaky clean.
Similarly, ammunition is manufactured so the distance from the base of the cartridge to the shoulder will always fit into a chamber. The tolerances of the loaded round and the chamber into which it fits are not allowed to overlap — the largest cartridge made must fit into the smallest chamber made.
Alas, not everything can be made to exact dimensions. So, the chamber is allowed to vary slightly. Generally speaking, the allowable “drift” of the headspace dimension is 0.006, or six-thousandths of an inch. If headspace is within that spread, it is deemed to be correct.
How is headspace measured?
When it comes to the AR-15 , almost nothing exceeds the volume of misinformation as does headspace.
If someone tells you that all you need is a USGI Field gauge, stop talking to that person right now. Stop listening to anything they have to say. Just walk away. Oh, there might be useful information there. After all, you have to paw through a lot of gravel to find a nugget of gold. And telling you that a Field gauge is the only tool you need is definitely gravel.
To measure headspace, there are three kinds of instruments. One is called a “Go/No-Go” gauge. Another is a set of precisely ground and measured gauges, each .001 inch difference in size. And finally, a micrometer that looks like a headspace gauge. Each works differently.
The Go/No-Go gauge is simple. The Go gauge is manufactured to the largest size of a cartridge. If you can close the bolt on a Go gauge, things are good, as the chamber is not below minimum.
The No-Go gauge is made to be a small amount larger than the largest allowable dimension of the chamber. If a bolt closes on a No-Go gauge, the chamber is too big. But, what does that really mean? When you fire a round, the case expands to fill the chamber. If the shoulder of the chamber is forward of the shoulder of the case, the case expands to fill the chamber. The shoulder must then blow forward to fill the gap. The rear goes back to the bolt face. In the middle, brass gets stretched.
Now, in the microsecond in which this happens, it can proceed in several different ways, but the end result is the same: the case gets stretched in the middle, weakening it. This matters only to reloaders.
Re-sizing brass pushes the shoulder back to minimum (this is called the “shoulder bump” and it ensures your loaded ammo will fit your rifles) and the next time you shoot, the brass will be stretched again. The effective service life of your brass depends on how much stretching, and how much bumping you subject it to. If your headspace headspace is within accepted limits, you can get 10 or more loadings out of your brass before you start seeing neck cracks from work-hardening. If the headspace is excessive, you may only get two or three.
An example of this is the SMLE in .303 British. The .303 headspaces on the rim, not the case shoulder. The British Army cared not a whit about reloadability, only for reliability. I gave up reloading ammo for my various SMLEs because I could not get cases to last more than three loadings in any of them.
The U.S. Army, Marines, Air Force, and let’s not forget the Coasties, do not care about reloadability of brass. As long as the brass holds together and is ejected in a single piece, they are happy. And that is where the Field gauge comes into play. The Field gauge has been developed with one thing in mind — how large can the chamber be and still have a rifle that can be used in a wartime situation? And that is why military armorers have a Field gauge that they use commonly, rarely reaching for their Go and No-Go gauges . An armorer may have a rack filled with rifles and carbines that would easily gobble up a No-Go gauge.
Use of the Field gauge only applied to used rifles and carbines. If a brand new, fresh out of the box unfired rifle is tested, and the bolt partially closes or locks on a Field gauge, it should be sent back, even in military use.
Why then is a Field gauge not a good choice for you? In short, you aren’t going to war. You will likely be using reloaded ammo. You have an interest in making that brass last as long as possible. If you need headspace gauges, get a Go/No-Go set.
What of the others? The “thousandths” set is used to determine not just that the headspace is within tolerance, but precisely what it measures. The micrometer gauge does the same thing, but instead of having to check the fit of the gauge set, you install/assemble the micrometer set, adjust, and then read the measurement.
The cognoscenti argue over the commercial .223 versus the mil-spec 5.56 headspace gauges. I talked to Dave Manson, a maker of headspace gauges about this. His quick reply was, “Which 5.56 set?” It seems there are a whole raft of gauges and specifications out there. And that is just in the shoulder location, not including the leade, which we’ll get into in a short bit.
How much can these vary? Let’s look at a few dimensions, hunted down and laid out for your curious gaze.
Source Go No-go Field Colt factory reject/aka Field II
SAAMI 1.4636” 1.4666” 1.4696” 1.4736”
USGI 1.4646” 1.4706” 1.4730”
So, if you have a rifle chamber just over the max size, call it 1.470 inches, and your sizing die is set to bump the shoulder back to fit under the minimum chamber size, let’s call that 1.460 inches, you are working your brass .010 inches on each shot. It isn’t going to last long.
How to check headspace on your AR-15
To measure headspace you’ll need a chamber brush, cleaning rod, bolt disassembly tool and a set of headspace gauges.
Unload your rifle and separate the upper from the lower. Use the chamber brush to scrub the chamber. Clean the bore with a patch to extricate any crud you scoured out of the chamber.
Remove the extractor from the bolt. Use the bolt fixture to remove the ejector. Scrub the bolt and dry it.
At this point, don’t be tempted to just drop the gauge in, close the bolt and see what happens. The closing bolt has enough mass and leverage to close the bolt on even a No-Go gauge if you are forceful enough or let it crash forward under full spring force.That is the wrong way to be doing it. The right way is as follows.
If you have the barrel out of the receiver, life is easy. Drop the Go gauge into the chamber. Hold the bolt by the tail, and see if you can insert it into the barrel extension, rotating it in front of the locking lugs. You should be able to do this. If not, that particular bolt/barrel combo is out of tolerance, under-minimum headspace and should not be used. Which is at fault? There is no way of telling with the tools at hand. To find out, you need at a bare minimum a surface plate, standing calipers and a bolt face cylinder. The cylinder is simply smaller in diameter than the bolt face opening, and a known thickness.
Put the cylinder on the surface plate. Stand the bolt on the cylinder, and measure the distance from the surface plate (which is flat to a millionth of an inch, if you bought the good one) and then consult the bolt drawing to see what that measurement should be. Without a bolt drawing there’s no real way to know.
If the bolt and chamber accepted the Go gauge, then remove the bolt and gauge, and replace the Go with the No-Go. Try again. The bolt should not, must not, rotate to the closed position. If it does, you have excessive headspace, and as before, you don’t really know which of the two is at fault.
A quick answer, in both instances is, both of them.
What if you have the barrel already installed in an upper receiver? Go to the store and buy yourself a foot of PVC tubing. You want a piece that has a quarter-inch inside diameter, or ID. The outside diameter, or OD doesn’t really matter since you can’t buy it with walls thick enough to not fit into the upper receiver.
Stuff the tail of the bolt into the PVC tube and use the tube as a handle to hold the bolt, as you insert it into the chamber while doing the Go/No-Go test.
How to adjust headspace on your AR-15
The first thing to do, if you have the gear and parts, is check the headspace on a rifle that has worked. If you are building a rifle from parts and do not have a working rifle to check, then you need to borrow one. Or go to the gun club with your parts and tools, and ask someone if you can measure theirs.
If the other rifle checks out, then send the parts back. If the other rifle doesn’t pass muster, then check your technique. The odds of two rifles from different sources both having incorrect headspace is exceedingly low.
Back in the early days, we ran into incorrect headspace frequently, as many people who were making parts were either new or not very good at it. I also suspect that a lot of the bad parts we found at sale prices in gun shows back then were production rejects, out of spec and should never have been sold. The bad parts makers got their names bandied about enough that they either improved or quit. Now, everyone who is still in the business knows the proper bolt or barrel extension dimensions. However, you may still find some parts that do not agree.
Brand new parts should fit. If you got the bolt and barrel from the same source, contact them and arrange an exchange. They should be happy to do so. If you got them from two sources, contact each and explain the situation. See which, if either of them, will help you. Accept their help and strike the other company from your list of “doing business with” for the future.
Only in extreme circumstances should you even think about chamber reaming to adjust headspace. And then only if the barrel is stainless or un-plated carbon steel and un-returnable. You can only correct insufficient headspace by reaming. If you have excess headspace, and you cannot return the barrel, then your only other option is a session with spare bolts and your headspace gauges. Maybe you’ll find a bolt that would otherwise be too big, but will solve your excess headspace problem.
I have reamed chambers and adjusted headspace in rifles without removing the barrel from the receiver, so it can be done. Having done it, I have to tell you this: remove the barrel. The hassle of reaming the chamber with the barrel in the receiver is greater than the hassle of removing the barrel and then reaming. Plus, you can do a better job with the barrel out. So, you’ll need the barrel and bolt, the finish reamer for 5.56 (not .223 Remington) with handle, cutting lube, chamber brush and a cleaning rod and patches for the bore once you are done.
Clamp the barrel, padded, upright in your bench vise. Scrub the chamber. Strip the bolt and scrub it clean. Check the headspace, just to remind yourself what under-minimum headspace feels like. (The bolt won’t rotate closed with the Go gauge in the chamber.)
Insert the reamer in your tap handle or reamer holder. Gently lower the reamer into the chamber, and begin rotating before it contacts the shoulder. Only turn in the direction of cutting, never reverse rotation. Give the reamer two full turns once you feel contact, pull it out while still rotating, and inspect the reamer. You should see metal chips on the shoulder of the reamer. If you also see chips on the body, you’re getting a bonus in the headspace adjustment; you’ve got a narrow chamber and the reamer is correcting that. You won’t have to mess around with small-base dies when you go to learn reloading.
Swab out the chamber and recheck headspace. With a few iterations you will get to the point where you can feel the bolt lugs start to cam underneath the lugs of the barrel extension.
Repeat the two-turn cutting procedure until the bolt will just rotate closed on the Go gauge. You have minimum headspace at this point.
You now have a decision to make. You can leave it at minimum, reassemble the barrel into a rifle again, and test fire it. You’ll probably find that it is plenty reliable, and unless you insist on shooting it in miserable environmental conditions it will serve you well.
If you want it to be a little more forgiving of neglect, you have to increase headspace past minimum, but not too much. Remember, the .006 inches of gap is all you have to work with.
Measure the overall length of your Go gauge. This is not the headspace, just the length of the gauge. Now carefully apply a small piece of tape to the base of the gauge and measure again.
You have just added a few thousandths to the Go headspace, by the thickness of the tape. Do the two-turn ream again, install the taped gauge and measure.
You can, if you are careful and diligent, add a controlled number of thousandths (adding .002, .003, or .004) to the minimum headspace, and not exceed the maximum.
Once you are done, scrub the chamber. Push a clean, dry patch down the bore to get lube and metal chips out of the bore, reassemble the rifle, and go to the range to test-fire and re-zero.
Why a 5.56 reamer, and not a .223? Because you want the leade to be 5.56. If your chamber is not only under minimum, but has a .223 leade, then the headspace reaming operation will take care of both.
If you have too much headspace, and your brass stretches, then what? Basically, subjecting the same brass to excessive headspace too many times will cause it to break in the middle. You probably won’t know it, because the broken rear half will be extracted and ejected. But the next round will find the front half crammed in the chamber. The fresh round will wedge hard into the remaining piece, locking the rifle up. You’ll have a heck of a time clearing the jam.
If you are using commercially reloaded ammo , it is possible for the brass to have been abused before it got to you. If it had been fired in something with grotesquely excessive headspace, like an M249 SAW, then it would have been stretched at that time.
So, a singular event, in reloaded ammo, may not be your rifle’s fault.But check anyway.