Headspace is the distance in your rifle’s chamber from the front of the bolt to the point at which the cartridge “headspaces” in the chamber. This description is confusing at first as it uses the term “headspace” to define what headspace means. A simpler way to think about it is this, there has to be some surface or point in the chamber which stops the cartridge case from going any further into the chamber. For standard centerfire rifle cartridges, the case is prevented from going any further forward when the shoulder of the case bears upon the interior of the chamber. For belted cartridges, the recess cut into the rear of the chamber that accepts the belt is the point that prevents the case from moving further forward. Rimmed cartridges are held in place by the rim. Whatever the bearing point is, the distance from the front of the bolt to the point at which the cartridge can go no further forward is the chamber’s headspace. Think of it this way, for a cartridge to be contained in a chamber, there has to be a stopping point at the front, the shoulder, and a stopping point at the back, the bolt face. The cartridge cannot go further forward than the shoulder, or belt, or rim, nor can it go any further backward than the bolt face. The distance between these two points is the headspace. For reference, the point on the shoulder at which headspace is measured is called the datum line.
So, why is headspace important? Headspace is important for reloaders to understand as the headspace dimension of cases must closely match the headspace of a firearm’s chamber to function properly. A cartridge case that is too long won’t chamber, or will be very difficult to chamber while a cartridge that is too short will move too far into the chamber out of reach of the firing pin and it won’t fire. When a cartridge fires, the brass case will stretch to fill the headspace, so a case that has been sized too short or a chamber that is too deep will result in rapid, excessive case stretching and eventual separation. Cases that are subjected to repeated over sizing and stretching will wear out in two or three firings at which point the head of the case will separate from the body just forward of the web. This is a dangerous situation as it can result in gases escaping from the chamber and being directed at the shooter. The ability to measure headspace of fired cases allows a reloader to size their cases just as little as necessary for the cases to fit into the rifle from which they will be fired. This is called partial full length resizing. Loading ammunition to closely fit the chamber of the rifle from which it will be fired can increase accuracy and case life. Ammunition that is sized and loaded to fit a particular firearm may or may not function in other firearms. If ammunition is being loaded for general use in several firearms of the same caliber, full length resizing is necessary.
It is a relatively simple matter to adjust your full length die to perform only minimal sizing on your once-fired cases. The most accurate way to perform minimal sizing on your cases is to use a headspace gauge. First, use the gauge to measure several of your fired cases. This measurement will tell you the headspace length of your rifle’s chamber. For minimal sizing in a bolt action rifle, set up your die to only size the cases down by .001 to .002. This minimal sizing will increase case life while potentially improving accuracy. Setting up your die will take a little trial and error. First, back it out of the press a full turn. Lube and size a fired case then measure the headspace dimension of the sized case. Continue screwing the die further into the press, sizing a case, and measuring until you have the die set up to reduce the headspace by the desired amount. When loading for semi-automatic, pump, or lever action guns, set up the die to reduce the headspace dimension by .005 for more reliable feeding and chambering. Due to the increased force applied to a round by the camming action of a bolt action rifle, rounds that just barely fit into the chamber should function properly. In other action types, minimal sizing may result in poor feeding and function. If you try partial full length sizing and begin to experience poor feeding, go back to complete full length sizing.
If you don’t have a headspace gauge, you can still set up your die for minimal sizing through trial and error. To do this, you will back out the die and then begin incrementally sizing cases until they are sized just enough to fit snugly in your rifle’s chamber. As you will be doing this by feel, it is helpful to remove the firing pin assembly from your bolt so that you will only feel the resistance of the case in the chamber. For this you will need several cases that have been fired in your rifle. Begin by backing the die out a full turn and size a case. Chamber the case in your rifle, feeling if there is any resistance. At this point, there will likely be little to no resistance as the case is not yet being sized any significant amount. Screw the die slightly further into the press, size another case, and then check it in the chamber. Repeat this procedure until you begin to feel resistance or are unable to chamber a case. At this point you have now sized the case enough to push the shoulder forward so far that the case is too long for your rifle’s headspace. The reason this happens, is that as the body of the case is sized back down, the case will stretch in length. At this point, the case needs to be sized further so that it will once again fit in the chamber. From now on, make very small adjustments to the die, screwing it in by small increments. Size another case and attempt to chamber it until the case chambers with slight resistance. At this point, the fit of the case is very tight in the chamber, but the camming force of the bolt is sufficient to chamber it. This is what is called a “crush” fit. Some reloaders like a crush fit, but for increased reliability and ease of chambering, it is advised to size just a little bit more to get the .001-.002 sizing. Screw the die in just a touch more and size another case, then check it in the chamber. Now, the cases should chamber more easily, but your die will be set up for minimal full length sizing and your cases will closely fit the interior dimensions of your chamber. Tighten down the lock ring and firmly tighten the lock ring screw so that this setting will be maintained. Once you have set your die up in this manner, there is no need to adjust it again unless the lock ring moves or you are going to load the same cartridge for another rifle. Setting up your dies this way will result in the minimum amount of sizing being performed on your brass which should increase the number of times you can load it before it begins to crack.
Understanding headspace can be a little difficult at first, but it is an important concept for reloaders to understand. Sizing your cases to closely match the headspace of your firearm will make your brass last longer and your rounds shoot more accurately. A headspace gauge or measurement tool can be very helpful to understanding the relationship between the size of your chamber and sizing die.