Headspacing 101

The very definition of headspace is expressed by many shooters and gunsmiths in different ways, almost as if there were some mystery attached. It being an esoteric term, I failed to find the word "headspace" in my Webster’s New Collegiate Dictionary. In fact, the word headspace does not appear in the glossary of shooting terms published by the National Shooting Sports Foundation. So here goes my interpretation of a word which I have understood and applied for more than a half century.

I define cartridge headspace as the space or distance between the case head and the bolt of breechface of a case that is fully and completely inserted to its most forward position in the chamber. A certain space or tolerance is needed between the cartridge case head and the bolt face or breech block to properly close the action.

This space or tolerance is established between firearm and ammunition manufacturers, Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI, saami.org) to permit ammunition of varying brands to be safely fired in firearms of different manufacture. Thus, the term cartridge headspace originated. Foreign made guns and ammunition must also conform to such standards. To further compound cartridge/ firearm headspace it must be understood that different methods are used to properly measure headspace.

By design, there are four basic classifications of cartridges where headspace is measured in different ways. These include the rimmed case, the rimless case, the belted case and the rimless, non-tapered pistol or rifle cartridge. I shall explain headspacing function of these four basic cartridge groups. Beginning with the rimmed style case, the rim or flange of the case head is larger than the major case body diameter. These would include cases like the .30-30 Winchester, .22 Hornet, .30-40 Krag, et al.

As the rimmed case is fully chambered the rim makes contact with the barrel as a stop. Some 300 Savage). Note how the case shoulder fits snugly against chamber shoulder wall as a means of attaining proper headspace on a rimless cartridge. guns have the barrel counterbored so the rimmed case head is fully enclosed within the barrel. Others may have the bolt counterbored to accept the case rim.

Either way, the cartridge rim is chambered firmly against the barrel as the bolt or breech block holds the case at proper headspace. The rimfire cartridge is also included in the rimmed cartridge family. Use of the rimmed case is an old and reliable form of cartridge headspacing. Of course, as cartridges are fired and reloaded, the case shoulder would also serve as a secondary headspace. Next is the rimless style cartridge.

These include a wide variety of cartridges like the .30-06 Springfield, .243 Winchester, .223 Remington, et al. While rimless cartridges do have a rim its diameter is the same or smaller than the case body. If a chamber for a rimless cartridge was reamed too deeply the case head would go too far into the barrel, resulting in excessive

headspace. Therefore, in the rimless case, a different means must be used to stop the case from chambering too far. Here the case shoulder or datum line is used as a stop in establishing correct cartridge headspace. Again, the firearms and ammunition manufacturers have established cartridge and chamber specifications so varying brands of ammunition fit varying chambers.

Thirdly, we will look at the belted cartridge which is often used on the Magnum cases like the .300 Weatherby, 7mm Remington Magnum and some others. In the belted cartridges the belt essentially replaces the rim in a rimmed style case for headspacing. The belt is substantially wider than the rim and set well forward of the extraction rim by about 0.220 inches. The belt serves as a cartridge stop for headspacing as the chamber is reamed.

As a belted case is fired, the shoulder (datum line) would actually serve as a secondary cartridge stop just as the rimless case shoulder does. In fact, handloaders sometimes need to full-length size the cases to permit re-chambering them. Lastly, we will review the rimless pistol type cartridge, which includes any of these like the .45 ACP and a few such cartridges also used in rifles. Here the mouth of the case, which is just marginally larger in diameter than the bullet, is used as a case stop or headspacing.

Firearms manufacturers and gunsmiths take on a large respon- From left, three case heads separated and a fourth with mid-body separation. The fifth case has its rim nearly blown off. Three sectioned cases at left show essentially normal appearance. sibility in making proper cartridge chambers. Of course, a reamer and headspace gauges are the key tools to accomplish this. The degree of precision in cutting a chamber helps to assure best accuracy in a rifle.

Perfectly coaxial reamer-to-bore alignment is the primary link to a good chamber. As I near maximum reamer depth when making a chamber, I test the headspace gauge, or an empty new cartridge, in the chamber. As I near go-gauge depth—perhaps 0.025" re-

maining—I carefully ream this part of the chamber by hand. You don’t want to over cut a chamber, so watch things closely at this point. As final headspacing is close, I torque the barrel down to about 75- 100 foot pounds to attain near optimal axial pre-load. Then, as the go-gauge chambers with a good feel on the bolt handle, the chamber is ready. This will satisfy the safety needs for factory ammunition and the handloader using conventional cartridges.

However, some of the improved and wildcat chambers will require special attention, particularly the rimless versions. Avoid a loose fitting cartridge in the wildcat or improved chambers. I prefer to use anew, unfired case in these chambers as a headspace guide prior to fire forming the cases. There should be firm neck-shoulder contact to avoid excessive headspace. In handloading, use of a fulllength sizing die can push the case shoulder too far and create excessive headspace.

Always approach this area of handloading with caution, particularly with rimless cases. Improper Cartridge Headspace The alert shooter should be aware of typical headspace problems. Avoiding these issues can lead to serious results, including blown up guns. Too short a chamber leads to difficulty in chambering and sometimes extraction problems. This can be avoided by cutting the chamber to proper depth in the first place or reaming the chamber slightly deeper until the go-gauge works correctly with some feel.

If ever in question about headspace have a qualified gunsmith check things out. Excessive headspace is often manifest by primer protrusion on firing. This is the result of a case being forced forward in the chamber by the firing pin. As case-body friction holds the case against the chamber wall, chamber pressure forces the primer rearward against the breech face.

Reloading and firing such cases While firing and closely monitoring a single case until case head separation ensued, bullet velocity and bullet impact were recorded. The first 14 shots were within the normal group. Then the bullet impact rose abruptly until the case head separated sending shot number 20 well above the group. All shots rose vertically with no appreciable velocity change between shot number one and number 20.

Some shots spiked as much as 125 feet per second above average. could cause case-head separation with disastrous results. The experienced shooter will be able to detect incipient case head separation and heed the warning. The telltale ring immediately forward of the case web is usually sufficient warning to the wise. A pointed hook inserted into the case mouth and dragged along the wall near the case head can reveal a case head stretching groove.

I won’t elaborate on a figure that constitutes excessive headspace in a firearm. Some feel an allowable tolerance of five to seven thousandths is tolerable. I feel any excessive headspace should be corrected. Setting the barrel shoulder back to proper headspace is one form of correction. A good way to check excessive headspace is to place case head-sized discs as shims between the case head and bolt face or headspace gauge and close the action. I use shims of about 0.003" for this test as needed.

In 1996 I conducted definitive research directly linked to the effects of bullet impact and velocity changes during actual case head stretching to the point of separation. A synopsis of this is shown in a captioned picture with this treatise. As we can see there are no real mysteries associated with cartridge headspacing. As part of normal function all firearms need some space between case head and bolt or breech block. The real culprit is excessive headspace as it makes its insidious entry.

It must be identified and properly dealt with, or shooting can become a major problem. AG