January 04, 2011
By P.O. Ackley
This article originally appeared in the June 1960 issue of Shooting Times.
By P.O. Ackley
1.Headspace for rimless rifle cartridges, such as the .30-06, is measured from the boltface to a point on the shoulder called the datum line. The datum line on the .30-06 case is the point on the shoulder that measures exactly .375 diameter.
2.On rimmed cases like the .30-30, the headspace is the thickness of the rim or the distance from the face of the fully locked bolt to the forward edge of the rim of the cartridge case that contacts the end of the barrel.
3. For the .375 H&H Magnum and other belted cases, the headspace is the distance from the face of the fully locked bolt to the forward edge of the belt.
4. Headspace for rimless pistol cartridges like the .45 ACP is measured from the face of the fully locked bolt to the end, or mouth, of the cartridge case.
A subject of great importance to the gunmaker or gunsmith who does any amount of barrel fitting or chambering is headspace. The gunsmith must have a complete understanding of this subject to instruct his customers in the proper use of handloading tools, especially for those interested in wildcat cartridges that they themselves make from some other type of brass.
Instead of attempting to define headspace, it will be dealt with here by describing each method by which headspace is measured.
There are four common methods of determining headspace, and these are closely related to the four general types of brass cartridge cases. The four ways are:
1. Measuring from the boltface to some point on the shoulder of the rimless case. This point on the shoulder is known as the datum line, and it is usually indicated on official data sheets or drawings. For example, the datum line on the .30-06 or .270 cartridge case is the point on the shoulder that measures exactly .375 diameter. The measurement varies with the size of the case; for example, the .25 Remington has a datum line of .392.
2. Rimmed cases on which the headspace is the thickness of the rim or the distance from the face of the fully locked bolt to the forward edge of the rim of the cartridge case that contacts the end of the barrel. The rim thickness varies with some rimmed cartridges, but normally it runs from .063 to .067. Notable examples of different cases are the R2 Lovell and the old .25-20 Single Shot, both of which have considerably thinner rims.
3. Belted cases, in which the headspace is the distance from the face of the fully locked bolt to the forward edge of the belt, which is .220 on the H&H Magnum cartridge.
4. Rimless pistol cartridges, usually straight and have no shoulder or rim to provide a stop, for which the headspace is measured from the face of the fully locked bolt to the end, or mouth, of the cartridge case. For example, the .45 ACP is a short, straight, rimless cartridge. When it enters the chamber, its forward motion is stopped by the end of the cartridge case coming into contact with the end of the chamber.
Wildcat cartridges were mentioned as being a problem to the gunsmith in conjunction to headspace.
Wildcat cartridges are not usually standardized to any degree, and headspace gauges are not always available for them. Even reliable chamber data often is not available. Usually reputable tool makers can furnish tools for any well-known wildcat cartridge, but very seldom do they furnish headspace gauges. Therefore, the gunsmith will find it necessary to determine the headspace himself as nearly as possible.
There will be considerable variation in the headspace of some wildcat cartridges as they are chambered by different gunsmiths, and it will be up to the owner of the rifle to make his own brass to fit his own individual chamber. It is often advisable for the gunsmith to provide a set of forming dies that have been adjusted and tested with the gun before it is delivered. Such dies can be of the plain hand type and of extremely simple design, but they can be designed in such a way to absolutely preclude the possibility of the owner of the rifle to produce cartridge cases for his gun with dangerous headspace.
If such forming and loading dies are furnished with the rifle to fit ordinary commercial loading tools, it will be necessary to instruct the customer in the proper use of the dies.
Many wildcat rifles are sold to customers who should never own such a gun--it seems to be impossible for some shooters to ever completely understand the problems of headspace. This results in a dangerous condition for the shooter and endless headaches for the gunsmith who built the rifle because invariably such shooters will blame the gunsmith for building a gun with excessive headspace.
This difficulty also occurs with some owners of standard caliber rifles: for example, some of the loading tool manufacturers make their sizing dies slightly short so that the headspace of the cartridge can be adjusted to fit the individual rifle. Other manufacturers furnish their full-length sizing dies on the long side so that it is impossible to set the cartridge shoulder back.
For the gunsmith, either method will result in headaches, for the owner of the long die will find it impossible to size old cases that he may have picked up on the shooting range so that they will enter his minimum-headspace rifle. The owner of the short die, on the other hand, will pick up such odd cases or use the fired cases from his own rifle and proceed to size them in his short sizing die, which he will insist on setting down tightly against the shellholder. This will result in setting the shoulder back to a point where headspace may be increased to a dangerous degree.
Some commercial dies can shorten the headspace of a rimless case as much as .020 or .025. It should be noted here that headspace is considered excessive if there is more than .006 space between the head of the fully chambered cartridge and the fully locked bolt. Thus, a cartridge .025 short on the headspace is apt to separate or rupture near the solid head and can result in a blown-up gun and injury to the shooter.
There are three general classes of cartridges that the gunmaker will be required to be familiar with and to work with. These are standard, improved, and wildcat.
There are also a great many problems that arise with the so-called improved cartridge. The word "improved" is a bad selection and does not accurately describe such cartridges, but this word has "stuck," and we are stuck with it.
A standard cartridge can be defined as one that is available commercially, commonly manufactured.
An improved cartridge is a factory cartridge that has been fired in an improved chamber and thus has its form changed. In other words, a rifle made to handle an improved cartridge, for example the Improved .257, will still handle factory ammunition, but the fireformed cases can be reloaded, or handloaded, to considerably higher velocities without danger to the shooter.
A wildcat cartridge cannot be obtained commercially or will not handle factory ammunition in any form. Wildcat cartridges must be made from easily-procured cartridge cases, such as .30-06 or .30-30. Some wildcat cartridges justify their existence. A few of them fill a gap that exists between certain commercial cartridges. Some do not justify their existence at all and have no advantage of any kind over corresponding factory cartridges. This is also true of so-called "improved" cartridges.
In rimmed cases, theoretically, there is no contact between the shoulder and the shoulder of the chamber. In fact, there is considerable space at this point, and when the cartridge is fired, the shoulder blows forward, or "fills out," and forms a solid contact. But this occurs only upon firing or when the pressure inside the case rises to a point sufficient to expand the brass cartridge case into all parts of the chamber.
The shoulder of belted cases does not contact the shoulder of the chamber and fits into the chamber exactly as described for the rimmed case.