Letters to this department reveal that one of the more serious problems that handloaders have to contend with is case separations. Most often the case head parts from the remainder of the case, but not always, because sometimes the separation takes place well forward of the case head. I’ve often detailed how to set the full-length sizing die and devoted a chapter to describing headspace in the 9th edition of my Practical Reloading Manual. But how is headspace actually determined?
Because the firing pin, which is projected through a small hole in the bolt face, must indent the primer sufficiently to produce ignition without rupturing or piercing, the cartridge must be firmly supported in the chamber at a carefully predetermined position. Since both ammunition and rifle chambers will vary slightly in production, due to necessary manufacturing tolerances, and both chambers and cartridge cases will contract and expand with temperature changes, a small but definitely limited space must be maintained between the bolt face and the head of a “maximum” cartridge. This small clearance is called “headspace.”
Basically, headspace is the distance between the face of the closed bolt or breech to the seating point that stops forward movement of the cartridge in the chamber. In conventional firearms, and depending on the type of cartridge, the seating point is at one of four positions. Rimless or semi-rimless handgun cases which have no shoulder and no body taper seat on the case mouth; rimless bottleneck cases seat on the case shoulder at an arbitrary point called the datum line; belted cases seat on the front face of the belt; and rimmed and some semi-rimless cases cases seat on the rear face of the chamber. The so-called semi-rimless cartridges are really rimmed cartridges with such a slightly protruding rim that they closely resemble true rimless cartridges, which likewise have a deeply cannelured extraction groove just forward of the head.
Headspace tolerances are established by gun manufacturers in cooperation with ammunition manufacturers. In the United States their efforts are coordinated by SAAMI (Sporting Arms and Ammunition Manufacturers Institute) which standardizes minimum chamber and maximum cartridge dimensions, to ensure that U.S- made ammunition of a given calibre will function in U.S-made guns of that calibre. European cartridges are standardized by CIP (Commission Internationale Permanente) located in Liege, Belgium. Countries which are signatories to CIP must have a government run proof house (banque d’ eppreuve) which must test and approve samples of all cartridges and must proof-test all firearms before they can be sold within any CIP country.
Headspace is measured with accurately ground, hardened steel headspace, or heading gauges made in both solid and adjustable styles. Their use demands a definite technique lest incorrect readings result.
Adjustable headspace gauges are used in the same way as solid gauges, but permit somewhat more precise measurement. They are used in conjunction with a micrometer caliper. Most headspace gauges are of the solid button or plug type and are normally furnished in sets of a “Go” and a “No-Go” gauge, sometimes with a third “Field” gauge. Intermediate gauges between the Go and No-Go gauges are also available where more precise measurement is required.
The bolt of a centrefire rifle should close on the Go gauge and should not close on the No-Go gauge, assuming that the firearm is new or has been only slightly used. The extremes indicated by these two gauges represent the normal parameters of headspace tolerance. However, after the gun has been subjected to great deal of use, particularly with high pressure handloads, normal wear, including a set-back of the locking lugs in their seats in the receiver can cause enough of an increase in headspace so that the bolt will close on the No-Go gauge. This does not mean that the gun has become unsafe. But some remedial gunsmithing is indicated when the bolt will close on the Field Gauge.
Excessive headspace to a degree, is not a dangerous condition in centrefire rifles of modern manufacture. Conversely a firearm is not necessarily safe to fire even when headspace is within the accepted tolerance.
Excessive headspace in centrefire rifles usually shows up in the form of incipient, partial or complete separation (rupturing) of the case forward of the solid head. In all types of centrefire arms, including pistols and revolvers, protruding primers may indicate excessive headspace, although not invariably so.
Excessive headspace can be very hazardous in rifles with receivers, bolts, or breechblocks heat-treated or case-hardened to a point where they become brittle. While such parts may have great tensile strength, they may also show low resistance to shock. A good example of this was an Eddystone M-17 action which blew into fragments during P.O Ackley’s tests carried out to determine the strength of military actions.
Due to increased end-play in the chamber, a cartridge with excessive headspace is not going to shoot as accurately as one that’s correctly headspaced. The firing pin blow is partly absorbed in driving the cartridge forward against its stopping point. Because the cartridge is not positioned uniformly in the chamber from shot to shot, ignition of primer and charge is likewise not going to be uniform.
Insuffient headspace may prevent closing the action completely on a factory cartridge or require undue force to close it. Excessive chamber pressure can result if the cartridge is forced into the chamber beyond its normal stopping point so that the case mouth is pinched into the bullet lede. Insufficient headspace may be corrected by deepening the chamber, by reaming or by removing metal from the face of the bolt.
Correcting excessive headspace requires more work. It can be done by fitting a replacement bolt or breechblock to gauge, or by setting back and rechambering the barrel. Adjusting headspace is a job for a skilled gunsmith equipped with the proper gauges, tooling and knowledge. Headspace gauges may be simple measuring instruments but the proper technique is necessary for their use. The rifle’s chamber, breech or bolt face, and action locking surfaces should be scrupulously clean to avoid interference from foreign matter.
In rifles equipped with hook extractors, the rim or flange of the gauge should be introduced under the extractor before inserting the gauge into the chamber. In a rifle that cocks on opening where the closing of the action is against mainspring tension it is advisable to remove the striker mechanism to increase sensitivity of the gauging.
After all these surfaces have been cleaned, the gauge is inserted into the chamber and an attempt made to close the action with light finger pressure only. Too much force should not be used as this may damage the gauge, rifle chamber, or gun operating parts, and will most likely falsify the reading. Turnbolt rifles in particular exert great camming power and it may thus be possible to close the bolt on the No-Go gauge and get a false reading.
Headspace tolerances in modern arms are very small. For a .30-06 cartridge tolerance is only .006. The length of the .30- 06 case from the face of the bolt to the datum line (the point on the shoulder that measures exactly .375 “ in diameter) is 1.94,” and the minimum headspace is held within the limits of 1.94” and 1.96”. The maximum headspace allowable therefore would be .006” in excess of the minimum measurement.
The use of shim stock of known thickness on the head of a cartridge case is not a reliable method for determining headspace, and should be used as a secondary measure only.
The man who reloads rimless rifle cases should be aware that full-length resizing of such cases may shorten the body of the case, creating a condition equivalent to excessive headspace. Such shortened cases may on firing show incipient, partial or in an extreme case, complete head separation as well as protruding primers - all signs of excessive headspace.
The fault may lie in the case sizing die or shellholder. Some commercial dies can shorten the headspace of a rimless case by as much as .020 or .025”. This is a dangerous situation because headspace is considered excessive if there is more than .006”space between the head of a chambered cartridge and the fully locked bolt.
Headspace may also become so excessive that the firing pin will not always reach the primer, and misfires will result; however, cartridges worked through the magazine will be held against then bolt face by the extractor and will fire regardless of the degree of excessive headspace. This is how .300 Magnum rifles have been blown up by .30-06 cartridges accidently introduced into their magazines and .30-06 rifles wrecked by a .308 Winchester round. Excess headspace allows the cartridge case to get a “running” start against the bolt face, and this may generate enough momentum to crack or shear the locking lugs.
A rimless-cartridge case which is considerably shorter than the chamber will be violently stretched from the centre toward both ends, frequently resulting in a complete separation, usually just in front of the head, which often wrecks the action and may injure the shooter. Pierced primers which flood the action with high-temperature gas, are quite likely to occur when the headspace is only moderately excessive. Only an idiot would attempt to shoot any ammunition in a rifle other than that for which the rifle is chambered.
Most die manufacturers recommend users to adjust the sizing die so that it just touches the shellholder to carry out full- length resizing. This is all very well, as long as the die and shellholder are correctly matched. It may result in an error however, if the die cavity is short or the case head recess in the shellholder is too shallow for the die used with it. Thus it is not a good idea not to mix dies and shellholders of different makes. The main thing to remember is never to set the die down against the shellholder until you are certain that the die and the chamber have the same dimensions at that setting.
A practical method is to check the resized cases in a full length case gauge like the Hornady Lock-N-Load Headspace Gauge Tool which allows the handloader to measure headspace from the case head to the datum line on a case shoulder with a dial/digital caliper. Five different bushing sizes insert into the Lock-N-load comparator body which attaches to a dial caliper and allows the handloader to measure the headspace on rimless cartridges from the .17 Rem. through the belted magnums. With this tool the handloader can measure changes between fired and sized brass and adjust his loading dies to achieve a proper fit in his chamber.
Many of the belted magnums pose a special problem because when the handloader knows that the case is supposed to be headspaced on its belt, he is prone to forget about shoulder-to- bolt face headspace. Then he may experience early case falure due to stretched or separated brass even with standard loads. This happens because such cartridges headspace on their belts and not a point on the case shoulder like rimless cartridges. Because head-to-shoulder length is not considered critical, many manufacturers of magnum rifles don’t bother to hold dimensions in the shoulder area of their chambers to close tolerances.
When the cartridge is fired in a sloppy chamber, the case shoulder is blown forward to match the chamber shoulder. Then when the case is full-length resized for reloading, the die sets the shoulder back to factory standard length. Each firing and resizing cycle stretches the brass just ahead of the belt, Often in as few as three firings, the case will develop an annular crack in the brass just ahead of the heavy interior head section. Usually the case holds together on firing, and then the extractor yanks the head of the case out of the chamber, leaving the body stuck in the chamber. The way to avoid this kind of thing is to either neck-resize belted cases, or adjust your sizing die so that case headspaces on the shoulder just like a rimless case.
It really doesn’t matter what the design of the case head is, if the case is not headspaced so that the shoulder makes contact with the shoulder in the chamber, or comes within .004” of touching, a stretch ring develops inside the case, leading eventually to case separation. To gain the longest possible case life, the shoulder of the case should contact the chamber shoulder firmly enough for slight resistance to be felt as the bolt is closed. Forget the rim or belt that is there to control headspace and set your full-length resizing die to headspace the case on the shoulder, the same as for rimless cases.
Headspace is measured from face of closed bolt or breech to seating or stopping point in chamber at one of 4 postions. (a) rimless or semi-rimless with little or no body taper seat on case mouth; (b) belted cases seat on front edge of belt; (c) rimmed and some semi-rimless cases seat on rear face of chamber; (d) rimless bottlenecked cases seat on case shoulder and headspace is measured from an arbitrary datum line. Arrows indicate headspace distance for each type of case.
Hunting is back on Australian televisions, with Series 3 of Beyond The Divide starting next week.