sions between the two will ordinarily not exceed a few thousandths of an inch.
Chambers and Chambering. The chambcr of a rifle, or the recess in the rear end of the barrel into which the cartridge enters, has a great deal to do with the accuracy of the arm. The reaming of chambers is one of the exacting operations of arms manufacture, calling for the utmost skill and care. While it is possible, with modern machinc tools, to chamber arms accurately and at the same time quite rapidly, nevertheless the work must be done by men who have had long experience with it. Perhaps it can best be said that the work of chambering should be done by men of long experience, for we sometimes encounter chambers that would reflect discredit on the village blacksmith. Naturally, the price of the rifle has something to do with this and a cheap arm that is hacked out to sell at a low price can not be expected to have the careful and painstaking workmanship that is put into the production of a more expensive one.
Chambers are made after the barrels are reamed and rifled, and they are formed by a scries of reaming operations. The breech of the barrel is drilled out to remove excess metal, after which one or more roughing reamers arc run into it to the proper depth, to bring the chamber approximately to shape. These first operations leave the chambcr too small at all points and do not go in to the full depth of the finished chamber. The character of the surface of the chambcr is of no importance at this stage.
The chambcr is brought to its finished size and shape by the use of additional reamers, each one of which removes only a small amount of metal. The difference between the finishing reamer and the one that precedes it is often little more than a thousandth of an inch. This final reaming 10 must be done with great care and with a carefully stoned reamer, in order to give that very smooth surface to the chamber which is so necessary to the easy extraction of fired cases.
So called straight chambers, like those for revolver cartridges, are the easiest to make, while bottlc-neck chambers for rimless cartridges offer the most difficulty; especially where the type of arm requires that the barrel be finished chambered before it is assembled to the receiver, as in lever action rifles. Barrels for bolt action rifles, as a rule, have the chambers left a few thousandths of an inch too short. After the barrel is assembled to the receiver, the chamber is reamed by hand to bring it to the proper depth with relation to the bolt, so that the head space will be correct. This head space reaming is confined almost entirely to chambers taking rimless cartridges and where the design of the rifle permits it, it can be done more precisely ajtcr the barrel is fitted to the receiver than before.
Chambering reamers, like all others, must be sharpened from time to time and this sharpening or stoning gradually reduces their size and, to a certain extent their shape until they can no longer be used. A finish reamer, which gives the chamber its final size and form, when worn our, is usually reduced in size and used on the next preceding operation, but between the time it is first put in use and the time it is worn out there is a diffcrcncc in the sizes of the chambers cut by it. Slight differences will exist without any change in the reamer itself, so that the production of two or more chambers that are exactly alike is a matter of chance. In addition, no two reamers are cxacdy alike, except by chance and different manufacturers may have different ideas as to the chamber form and taper they wish to use, which is influenced by the nature of the arm being made. Ordinarily, the greater the taper of the chamber, the easier the extraction of fired cartridge cases will be and a litde more taper is necessary in the chamber of an arm I j having a limited amount of power or leverage for extracting, than in a bolt action rifle having a powerful camming action for the extraction of fired eases.
It is not the intention to go into the details and problems of chambering here, but the reloader should understand that chambers of the same caliber differ considerably between makes and models of arms and also, to a lesser extent, in arms of the same make and model. It should also be understood that the mere fact that a chamber appears to be large and permits a visible expansion of the cases fired in it, docs not mean that the arm is poorly chambered. Such a chambcr may be necessary to the proper functioning of that particular arm and is to be distinguished from a poor chambcr.
While the chamber proper supports the walls of the cartridge case against the severe stresses incident to firing the cartridge, the total over-all length of the chambcr, in relation to its cartridge, is governed by the bolt or breech block that closes it and supports the head of the cartridge case. In other words, the location of the face of the bolt or breech block, governs the head space of the arm.
The head space of an arm is the distance from the surface of the chamber or barrel, that positions or prevents further forward movement of the cartridge into the chamber, to the face of the bolt or breech block when the latter is fully back against the shoulder that supports it. Rimmed cases are positioned by the rim which bears against the rear face of the barrel or, in the case of revolvers, against the rear surface of the cylinder.
Rimless cases present a special problem for, as their name indicates, they have no rims to act as a stop against their forward movement into the chamber. The shoulder of the case serves this purpose, therefore the head space of a rifle for a rimless cartridge is the distance from the beginning of the shoulder of the chamber to the face of the bolt. The measurement of head space is taken from die beginning 13 of the shoulder, because the angle of the chamber shoulder and the angle of the shoulder of the cartridge case are not the same, the former being the less abrupt of the two.
There is usually a small amount of play between the face of the bolt and the rear of a rimmed cartridge, when the latter is in the chambcr. The clearance must be sufficient to take cases of maximum rim dimension, plus a small allowance for the occasional thick rimmed case that will always get by the inspectors once in a while. Consequently, with cases of a minimum rim thickness there will be several thousandths of an inch clearance between the head of the case and the bolt. The rim of the case, being of solid metal, can not be compressed and if the fitting of the bolt were too close, it would frequendy be impossible to close it on the cartridge.
The situation is different with rimless cases. The head space may be greater than the shoulder to head length of the cartridge but it may also be, and frequently is, less. The boh will close on a rimless case that is longer than its chamber, bccause there is an opportunity for the shoulder to give slightly under the pressure of the bolt, or the ease walls may spring out slightly, or both. Furthermore, as a new cartridge is always smaller than its chamber, its forward movement into the chamber is not stopped precisely at the beginning of the chamber shoulder. For these reasons, the head-to-shoulder length of the cartridge may be greater than the corresponding length of the chamber and still have the arm function satisfactorily.
When, due to the set-back or wear of the locking surfaces of a bolt or breech block, the head space exceeds the maximum limit set by the manufacturer of the arm, the arm is said to have excess head space. Many people are under the impression that "exccss head space" indicates a dangerous condition, just because a few arms having this condition to an abnormal degree have been known to blow up. This idea is fallacious, for it is obvious that no rcputable manufacturer is going to put out arms that are on the ragged edge of being dangerous. The maximum limits of head space for all rifles are established so as to leave a very liberal margin for any increase that is likely to occur through ordinary usage.
Influence of Head Space on the Case. The diagram on this page shows the points from which head space measurements are taken for both rimmed and rimless cartridges. It will be observed that most of the solid head of the rimmed case is well within the chamber, while only a small part of the solid head of the rimless case enters the chamber. It will also be noted that the nccks of the cases (new cases) do not reach the forward end of the chambers. This clearance is provided to insure proper functioning of the arm, even if an occasional ease of extra length is loaded into the chamber. It also provides for smooth operation in spite of any minor fouling of the chamber.
When a cartridge is fired and the burning powder begins to build up pressure in the chamber, the thin walls of the cue expand, gripping the walls of the chamber. If there be toy excess head space, the blow of the firing pin will usually lhc cartridge forward, leaving a space between the roe of the bolt and the cartridge head. With the walls of
14 the case gripping the chamber walls, the head of the case will be driven back against the bolt. This will stretch the brass to a greater or lesser extent, depending upon the distance that the head moves. The point of strain is usually at about the location of A and B on the sketch, although it may occur further forward. This stretching will thin down the brass in the walls of the case and weaken it and if the excess head space or the movement of the case head to the rear be great enough, a complete head separation will occur. When there is a partial or complete separation of the head of a case at high pressure, there is a possibility of injury to the arm, the shooter, or both, but often there is enough of the side wall left to act as an obturator and stop most of the gas. This is especially true of the rimmed type of case, which has practically all of the solid head within the chamber. Such gas as may escape to the rear is deflected by the rim of the case, so the separation of the head of a rimmed case seldom results in injury of any kind to the arm or to the shooter.
With rimless cases, the situation is not so good. The point where the head usually separates is so close to the end of the chambcr that the likelihood of gas escaping to the rear is much greater than with the rimmed case. In addition, the 13 rimless case has no rim to deflect this gas, which, in bolt action rifles, will come back through the bolt, causing eye burns or perhaps more serious injury, accompanied by the wrecking of the rifle. This can also happen with some lever actioa rifles, except that one is relatively safe from eye burns with rifles having solid or completely enclosed actions. However, such rifles are harder to head space accurately and when a head separation is accompanicd by the escapage of a considerable amount of gas at high pressure, the shooter is in for trouble regardless of the kind of rifle he is using, for no arm is safer than the cartridge cases used in it.
Assuming that the head space is not great enough to cause a head separation, the cartridge case will be stretched and expanded to fit the chamber perfectly. If it is not resized
15 or if it is only resized at the neck, the head will be in perfect contact with the bolt the next time it is fired and there will be litde or no further stretching of the case. Naturally, excess head space will cause the case to lengthen, will reduce the thickness of the side walls near the head and there is no way of returning the strained part of the case to its original condition. If the case is resized full length, it will merely set the shoulder back the same distance that the case stretched and will give the case the same clearance between die head and the bolt that it had originally. When fired again, the head will set back again and the side wall will be further weakened or may even tear apart. Mercuric primers will aggravate this condition, as the mercury will penetrate the strained brass rapidly and render it brittle and useless, even at low pressures.
If you have a rifle in which head separations occur with factory loaded ammunition, it is a pretty good indication that the arm has an excessive and dangerous amount of head space and you should communicate at once with the manufacturer regarding its repair or adjustment.
Cartridge cases loaded with high pressure loads will lengthen even though the bolt or breech block be in firm contact with the head of the case. The brass is forced
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