Tips
From Bryce Handloading - Headspacing |
| Headspacing
is a commonly misunderstood term. So are the problems related to headspacing.
A lot of shooters are even more confused than my buddy who said, “I
always thought headspacing was something we did in the sixties while listening
to Jimi Hendrix.” Simply put, headspacing is the distance between the face of the firearm’s bolt or breech face where it contacts the head of the cartridge case and the location in the chamber where the case is supported against further forward movement. The term probably originated back when rimmed cartridges were all the rage. A rimmed cartridge is supported by the rim and so the headspacing distance is the thickness of the rim. Or, in some dialects, the “head” of the case. Today we have several different types of cartridge cases and they use different methods to headspace. With belted cases, the belt is often used to support the case. So the headspacing distance is from the face of the bolt or breech face to the part of the chamber that contacts the forward edge of the cartridge belt. With a rimless or rebated rim bottleneck cartridge, the shoulder of the cartridge contacts the chamber and supports the case. With this style of case, the headspacing distance is from the face of the bolt to the part of the chamber that contacts the shoulder of the case. Because the shoulder of a bottle neck cartridge is an angle, the headspacing measurement is taken from a specific location on the shoulder known as the datum line. Straight walled, rimless cases such as the .45 ACP headspace off the case mouth and the headspacing dimension is from the front of the bolt face to the part of the chamber contacting the mouth of the case. Each chamber in a firearm was created with a defined set of manufacturing tolerances within which the headspacing dimension must fall. That usually varies by about .007-inch. But, a quick check of the SAAMI book shows that’s not always the case. For example, both the .35 Whelen and the .358 Winchester both have a tolerance of .010-inch in the chamber. So the actual headspacing dimension can vary by as much as a tenth of an inch from the defined measurement. Also consider that ammo manufacturing has a similar acceptable tolerance variation. So if you have a chamber at the long end of the acceptable tolerance and ammo at the short end of the tolerance you may have an excessive “headspacing” condition close to .020-inch and still be within acceptable tolerances. Headspace can be measured in a firearm with a set of headspacing gauges. The set consists of a go and a no-go gauge. To use them you simply put them in the firearm like a cartridge and gently try to close the action. It should close on a go gauge and not close on the no-go gauge. If the action will not close on the go gauge or will close on the no-go gauge you have a firearm with a headspacing problem and should take it to a competent gunsmith for repair. A short headspacing condition will prevent properly chambering a cartridge. Excessive headspace can cause misfires and poor accuracy. With bottleneck cartridges it will also lead to poor case life and, in extreme examples, dangerous rupturing of the cartridge case when firing. One common problem is that the case will stretch just ahead of the web and upon subsequent firings the case will split along this stretch point. Conditions that many handloaders assume are headspacing problems are in fact not. Perhaps the most common is that the handloader is adjusting the resizing die to make hard contact with the shell holder and pushing the shoulder back too far on the case. Resizing dies also have a tolerance range when manufactured and like anything else, the quality control can vary. With some dies, full-length resizing the case can actually push the shoulder back on the cartridge case too far and create an excessive headspacing condition, even though the firearm’s chamber is well within specifications. If factory loads work fine in your gun, but you are having poor case life, poor accuracy or misfires with your handloads, you may be pushing the shoulder too far back on your handloads. |
|