What is Headspace?  How do we help?

Introduction

To recognize the benefits of Headspace Control Shims^TM we must first understand the problem of headspace in firearms, which can be minimized with reloaded ammunition assembled using Headspace Control Shims^TM and standard reloading equipment.

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Headspace and Related Problems

Headspace refers to the gap between the bolt or breech block of the firearm, and the head of the cartridge case (case) when it is in the extreme forward position of the firearm's chamber (figure 1. "Pre-Ignition"). Headspace diminishes accuracy and shortens the reloadable case life.

During the pre-ignition phase of firing a gun, the firing pin hits the primer of the cartridge. This drives the cartridge to the extreme forward position of the gun's chamber. Immediately there after the firing pin detonates the primer which ignites the gunpowder in the cartridge. Expanding gasses from the burning gunpowder generate high pressure inside the case. The pressure causes the section of the case just forward of the case head to stretch, and the case head driven back until no headspace exists (figure 2. "Ignition"). Then the bullet is projected out the barrel leaving behind the case and spent primer. The case is now "fire formed", and has an optimum fit to that gun's chamber. The case stretching diminishes accuracy and thins the case wall around the inner case perimeter, forming a headspace ring just forward of the case head, which shortens the reloadable life of the case. Fire formed cases offer the greatest degree of accuracy and the longest reloadable case life, provided the fire formed case is not excessively resized during the reloading process. Excessive resizing would introduce a headspace condition again and repeat the aforementioned cycle upon firing of the reloaded cartridge.
Headspace normally exists to some extent in all firearms. This stems from the fact that all firearms manufacturers have an acceptable size tolerance to make the gun's chamber, where as the ammunition manufacturers have a smaller size tolerance to make their loaded ammunition to. These tolerances must be compatible so that the largest allowable cartridge size will fit into the smallest allowable chamber size. We should expect to see a small headspace condition with this combination. Conversely, the greatest headspace condition would result with the smallest allowable cartridge size combined with the largest allowable chamber size.

The benefit of minimizing headspace is then to optimize accuracy regarding headspace, and to prolong the reloadable life of cases.

The Reloading Process

The reloading process involves resizing the empty brass case so that it will be just slightly smaller than the fire formed condition. Slightly reducing the case's size (resizing) is required to permit the case to hold a new bullet, and eases insertion of the reloaded cartridge into the gun's chamber. One popular resizing method is "full length resizing" which is designed to resize the brass cartridge case to the commercial cartridge case tolerances. This process negates the accuracy benefit of reloading regarding headspace as it oversizes the case and recreates the headspace condition.

To setup reloading equipment to full length resize, it is necessary to have the shell holder contact or "bump" the base of sizing die when the press' ram is in the uppermost position (figure 3). The tightened working position of the die is regulated by the position of the lock ring† on the threaded exterior body of the die. Although the lock ring's position is adjustable along the die; it is a timely, cumbersome, and imprecise operation. This has placed the reloading community without an effective means to quickly, and/or precisely change the working position of reloading dies.

Usefulness

The usefulness of Headspace Control Shims^TM is that it enables the user to quickly, precisely, and manipulate with repeatability the working position of reloading dies or equipment along the longitudinal axis of the die without the timely and inaccurate process of repositioning the die Lock-Ring† .

By changing the die positions the hand loader can make precise adjustments to resizing operations in an effort to minimize case sizing and eliminate headspace. Precise adjustments can also be made to bullet seating operations enabling optimal bullet seating depth. Four hundred ninety eight (498) unique precisely incremented positions can be achieved by simply changing the shim thickness and/or the shim stack configuration between the die Lock-ring† and the Reloading Press*. Additionally, if the user has more than one Reloading Press*, Headspace Control Shims^TM enable the user to interchange each set of reloading dies with virtually all‡ Reloading Presses*. This would be accomplished by setting the dies to function correctly in the reloading press having the greatest "shell holder-to-lock ring bearing surface" distance, "G", measured at the uppermost position of the reloading press' stroke. Dies could then be shimmed in all‡ other presses to account for the differences in the presses, "D", and further more could be shimmed to achieve the optimum reloading conditions in each reloading press.

Glossary

* - The terms "Reloading Press" and "Reloading Presses" are intended to include any: "C" frame reloading presses, "O" frame reloading presses, "Turret Head" reloading presses, Single Stage reloading presses, Progressive or Multi-Station Turret reloading presses, all Multi-Station or Progressive reloading presses, Die Platforms, Toolheads for use in Multi-Station or Progressive reloading presses, Turrets for use in Multi-Station or Progressive reloading presses, Lock-N-Load bushings, Lock-N-Load Classic and Auto Progressive reloading presses, and any piece of reloading equipment intended to accept and use reloading dies.

^† - Any Lock Ring, Die Lock Ring, Steel Lock Ring, Die Lock Nut, Stop Ring or any device intended to bear on the Reloading Press* to repeatedly establish a working position for the reloading die or reloading equipment along the longitudinal axis of the die.

^‡ - Headspace Control Shims^TM are not designed or intended for use with the Forster/Bonanza CO-AX Press.

Shell holder - the term "shell holder" is intended to include any: standard single stage shell holder/s, Competition shell holder/s, and any shell plate used in a progressive reloading press.

Die - the terms "die", or "dies" describe any single, group, or combination of: neck sizing die, bushing die, full length sizing die, small base sizing die, forming die, reforming die, custom die, bullet seating die, trim die, competition size die, competition bullet seating die, bench rest size die, or bench rest seating die.