CAUTION: When purchasing headspace gages make SURE you get the proper gagues for YOUR rifle. In other words, if your rifle is chambered for .223 Remington, get .223 Remingon gages NOT 5.56mm gages. Conversely, if your rifle is chambered for 5.56mm, get 5.56mm gages, NOT .223 Remington gages. There IS a difference. See 223 vs 5.56mm Headspace is that measurement describing the size of the chamber in a barrel. In the case of rimless rifle cartridges, it is the distance from some arbitrary point on the case neck taper back to the bolt face.



In the more general case (e.g. pistol, rimfire, rimmed cases, and belted magnums), you can imagine headspace as measured from whatever the cartridge rests against on the front of the chamber all the way back to the bolt face. These numbers are specified for each cartridge by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) and it's important to have this measurement fall within SAAMI specified tolerances. Too small, and you may not be able to close the bolt on some ammunition that's on the large side of its tolerance range. Worse yet, it will close and let you fire. Firing ammunition in a chamber that's too tight leads to dangerously high pressures. Too large a chamber, and there's lots of room for the cartridge to rattle around in there -- well not really. What happens is that the brass casing can rupture and 55,000 psi hot gases start rushing out in every direction, including at your precious body parts. Think of the high pressure gases that force a bullet down the barrel finding other avenues of escape. This is what some people mean by "the gun blew up on me". One sign that this is going to happen soon is that the backs of your brass casings start looking like someone took a sledge hammer to them and flattened out the primers and lettering. This is because the casing is literally being hammered against the bolt face. This can also be caused in reloads by packing the cartridges with too hot of a load.

So that's headspace, and why it's important. But what are all these gages about? They look like little steel cartridges without the bullets. You stick them into the chamber and try to close the bolt. Whether the bolt closes or not -- and whether that's good or bad -- depends upon which gage you're trying to use.

Using Headspace Gages It is important that you use the headspace gages properly. Used improperly, the gages will give false and possibly dangerous information. For example, if you place a NO-GO gage in the chamber, pull the charging handle all the way back and let it fly, the bolt may well close on the gage due to the buffer spring pressure. However, done properly, e.g. closing the bolt by hand and trying to turn the bolt, the bolt will not close without excessive hand pressure.

GO GAGE This gage is used to determine proper headspace. Insert the "GO" gage in the chamber, then take the bolt, without the extractor or pin ejector and insert it and turn. The bolt should turn on the gage with some force. If the bolt does not turn on this gage, the chamber is not reamed to the correct depth or the bolt is oversized. There is inadequate headspace. Thus, if you can close the bolt, it's a "GO" on the barrel/bolt combination.

NO-GO GAGE This gage is also used to determine proper headspace. Insert the "NO-GO" gage into the chamber, then take the bolt, without the extractor or pin ejector, insert it into the extension and turn. The bolt should not turn on this gage. Do not force the bolt. If the bolt turns on this gage, the chamber is too deep or the bolt is undersized. There is excessive headspace. Thus, if you can close the bolt, it's a "NO-GO" on the barrel/bolt combination.

FIELD GAGE The GO and NO-GO gages are used to check the limits of factory tolerances between the chamber and bolt to ensure that tolerance build-up is neither inadequate nor excessive. The "FIELD" gage, checks for headspace in excess of the factory tolerances, thus the gage is used to determine excessive headspace. Insert the "FIELD" gage into the chamber, then take the bolt, without the extractor or pin ejector, insert it into the extension and turn. The bolt should not turn on this gage under any circumstances. If the bolt turns on this gage, there is excessive headspace and the WEAPON IS VERY DANGEROUS.

Fortunately, since the wear mechanism for a firearm chamber is to stretch, we only have to worry about the dimension beyond the NO-GO measurement; which is our "field" gage. The field gage is sized to be the largest acceptable headspace possible. If you can close the bolt on the field gage, then the barrel is worn out and it's time to replace it. To continue shooting it is dangerous.

In theory, the bolt of a well used rifle will close on a NO-GO gage too. UNDER NO CIRCUMSTANCES SHOULD A BOLT CLOSE ON A "FIELD" GAGE. That's when you rebarrel the gun.

Headspace is another matter that needs to be considered when reloading ammunition. In my opinion, it is essential to use a full length resizer die on the cases. This assures that your reloaded ammunition will work in ANY firearm, not just your rifle. The cases stretch and conform to the chamber they were last fired in and if they conformed to a chamber different than yours (and they did) and you don't full length resize, you may be developing inordinately high chamber pressures, which may lead to a very bad day. I also recommend using a factory crimp die in lieu of a standard roll crimp die. This makes reloading life somewhat easier, since you don't have to trim all the cases to the same length, but you still have to trim cases that are over the maximum.

Since you've got a new gun (with less than 10,000 rounds through it), I would suggest you buy both a GO and a NO-GO gage. Play it conservative, check the headspace every few thousand rounds and don't go beyond the NO-GO. As you know, most people don't even bother with that but I like to play it real safe.

Basic rules.... For a new gun: a. Closes on the go gage b. doesn't close on the no-go gage For an old rifle that's been rode hard and put up wet: a. closes on the go gage b. probably closes on the no-go gage c. definitely does NOT close on the field gage

It's cheap insurance to have a set around for every rifle you have, especially if you shoot a lot. If you plan on changing bolts and/or barrels around, you have to check each combination because each setup could vary a few thousandths. Fortunately you only have to do it when you first introduce them to each other, thereafter, every few thousand rounds if you wish.