What follows is not intended to be a dull, boring treatise about a dull, boring topic. My intent is to provide clear and concise insight into an important safety and reliability factor that a competent handloader will consider when preparing brass for reloading.
So what is headspace, why is it important, and how does one ensure achieving proper headspace when making safe and reliable handloads? A practical, descriptive definition of headspace is how well the cartridge fits the gun’s chamber.
Obviously, handloads must have the correct and reliable energetic components (primers and propellant). The cartridge case must have a tight primer pocket, proper size flash hole, adequate neck tension, and no defects that might cause it to rupture and leak hot gases when the round is fired.
Another, less obvious, criterion you must consider to ensure your ammo will fire when you squeeze the trigger is headspace. In order for the primer to ignite the round, it must receive an appropriate strike force from the firing pin. The impact crushes the explosive pellet between the primer cup and anvil to ignite the powder charge.
The required strike force depends on several things: firing pin spring force, firing pin shape, how much it protrudes beyond the breech/boltface, and headspace. If the spring is strong enough, the firing pin is not damaged or too short, and the gap from the cartridge case head and the breech/boltface is not too far, you can count on reliable and consistent ignition. However, if that gap is too great, you will likely experience a hangfire or even a misfire.
Assuming all other factors are within spec and the chamber has been cut properly, the cartridge case design and actual case dimensions control whether you have proper headspace. There are several different cartridge case design variations, including rimmed, belted, rimless bottleneck, and straight-walled. The headspace of each type is dependent on one critical case feature.
The rimmed case (.30-30 Winchester for example) is the simplest. The chamber is counterbored to allow the rim diameter and thickness to be fully enclosed. I reformed and trimmed some .405 Winchester cases and loaded a box of .35 Winchester ammo for another writer; however, my carefully crafted handloads wouldn’t chamber in his vintage Winchester Model 1895 lever action. That’s when I discovered the .405 Win. rim is 0.012 inch thicker than the .35 Win. rim.
A belted case headspace is essentially achieved in the same manner as a rimmed case. Again, the chamber is counterbored to accept the belt to properly position the round. However, many rimmed and belted rifle rounds are also bottlenecked, so it’s good practice to follow the same steps described in the next paragraph to ensure maximum case life when reloading both types.
Rimless bottleneck cartridges headspace on a specified datum located on the case shoulder. Think of the fit between these cartridge cases and chamber this way: The case shoulder travels forward until it touches the chamber shoulder. When the action is closed, whatever gap remains between the case head and the breech/boltface is the actual headspace tolerance. The competent handloader will adjust the full-length sizer die so that it just kisses the case shoulder and doesn’t set it back more than a couple thousandths of an inch.
If the bottleneck case shoulder is set back too far when it’s resized, the case will have excess room to stretch as it expands under pressure to fill and seal the chamber. After several loading/firing cycles, the case wall just ahead of the case head will eventually work harden and then crack.
When you’re reloading any cartridge case, you must also pay attention to case length. Each time a round is fired, resized, and reloaded it will stretch. Doing this over and over, it will stretch until the maximum case length is exceeded. If that occurs, the round may or may not chamber, and it’s surely not a headspace issue. If you’re lucky, it won’t chamber and you’ll have an opportunity to discover the actual problem without a dramatic consequence.
However, if you force the too-long case into the chamber, you may cause the chamber throat to crimp the case mouth into the bullet shank. When you squeeze the trigger, you will likely experience a potentially serious mishap. Because the case mouth can’t expand and release the bullet, excessive pressure builds up until it forces the bullet out of the case. The primer may blow out of the primer pocket, and the case can rupture. Not a good scenario!
Some handloaders only neck size a bottleneck case if it will be fired again and again in the same firearm. Optimal headspace is maintained because the case body and shoulder are already fireformed to the gun’s unique chamber and only the neck is resized to hold the bullet in place.
straight-walled rimless case headspaces on the case mouth, so the case length is the critical feature. Popular pistol rounds like the 9mm Luger, .40 S&W, and .45 ACP fall into this category. The .450 Bushmaster and .350 Legend are examples of rifle cartridges that headspace on the case mouth. Of course, some straight-walled cases are also tapered, which actually helps achieve the proper chamber position. In my experience, straight-walled rimless brass doesn’t stretch very much, but you still should spot-check a few cases after resizing just to be sure.
This short discussion is intended to help you understand the importance of how your handloads must fit your gun’s chamber to ensure safe and reliable function. For more information, several of the full-service reloading manuals (Hornady, Speer, and Lyman) include comprehensive narratives on how to achieve proper headspace when reloading.