Headspace

Get Paid To Write Online

Get Paid to Write at Home

Get Instant Access

forward and while this docs not ordinarily result in any weakening of the case, dicre arc exceptions to this general rule. It may be a matter of chance or it may be due to soft spots in the case but occasionally a case wall will be weakened from this cause. The strain, if any, may occur anywhere from the shoulder back to the head of the case. A few years ago, the writer conducted a series of experiments for the Cuban Army, in order to determine the approximate rate of elongation of the .30 06 cartridge case. Some .30-06 ammunition was fired in a rifle having the minimum head space of 1.940 inches, then ammunition from the same lot was also fired in another rifle having the maximum head »pace of 1.946 inches. The cartridge cases from each rifle were kept separate and were reloaded with the Model 1906 bullet and a powder charge developing 3,700 f.s. muzzle velocity, at a pressure of 49,000 lbs. per square inch. After each firing, the cases (each one stamped with an identifying mark) were measured for increases in length and were resized so the body length was 1.9487 inch.

The cases fired in the rifle having minimum head space showed an average elongation of .027 inch after four rcload-ings plus the original firing, while those fired in the rifle with maximum head space showed an average elongation of .035 inch with the same amount of firing. The only case which showed any localized weakening was thin as paper at the shoulder, but this case might have been thin at this point originally. Practically all of the cases were increased in length so their mouths were jammed into the forward end of the chamber after the fourth reloading but, with the one exception mentioned, all could have been made serviceable again by trimming back to their original length.

Cases will elongate when reloaded with full charges. If this elongation prevents the free entrance of the case into the chamber, the mouth of die case urn, without harm, be filed or reamed enough to shorten it slighdy. Reamed is preferable.

Some rifles have bolts or fairly long breech blocks that lock at the rear end, instead of at the front immediately back of the cartridge. Such arms, while perfectly safe, permit cases to stretch noticeably when fired with high pressure loads. The bolts, being supported at the rear, have a tendency to spring or buckle slighdy under the thrust of the cartridge head and it is not infrequent that a high power cartridge case from such an arm will fail to enter its own chamber again, at least it will not go in far enough to permit the action to be dosed. This stretching produces a condition similar to that caused by the presence of excess head space and where this condition is encountered, it is advisable not to reload the fired cases with full loads. The cases must be entirely resized to be used and with full loads the stretching will be repeated, which may lead to head separations.

Expansion of Cartridge Cases. It has been pointed 17 out that cartridge cases of the same caliber will vary slighdy in their dimensions and that chambers of the same caliber will also vary. These variations are remarkably small but they do exist. It is obvious that the smallest chamber of any given caliber must be large enough to take the largest cartridge made for it and, conversely, in manufacturing the ammunition, the maximum cartridge must be kept within the size of a minimum chambcr of that caliber. When a maximum cartridge is put into a minimum chamber there will be a small clearance between the two, that is, the un-fired cartridge will enter and extract from the chamber freely. However, when a minimum cartridge is put into the same chambcr the clearance is greater and, if we make the same comparison with a maximum chambcr it is easily seen that the clearance between cartridge and chamber is still further increased.

When a cartridge is fired, the internal gas pressure forces the walls of the case firmly against the chamber walls and after the bullet has left the bore and the pressure drops to zero, the walls of the case do not go back to their original position. The case will have taken on a permanent set and expanded to pcrfccdy fit the particular chamber it was fired in. The springy nature of cartridge cases cause them to spring back slighdy after the pressure drops so that they may be extracted easily and in most instances they will reenter the same chambcr without difficulty. As already explained, if there is spring or set back in the bolt or breech block, the case may elongate in a way that will prevent its re-entering the same chamber again without resizing, but elongation and lateral expansion arc two different things and they affect the cartridge case differently.

In aay normal commercial or military chamber the ex~ pansion that takes place in the cartridge case is unimportant, except that it is improved in as much as it is now a "tailor made" case, fitting its particular chamber more perfectly than it could be made to fit by any other means. If it has not 18 been weakened by stretching, the case may be reloaded with charges approximating the original factory charge, or at least the original factory pressure. On the other hand, if either the stretching or the expansion is excessive at any point, the case should be used only for reduced loads, or discarded if the conditiuu is bad enough.

Split AJecks. Excessive expansion at the neck may causc a split or opening to occur at that point. Occasionally, and particularly in old ammunition, cartridges may be found with the necks split, due to season cracking. These split necks may be visible or they may split under the stress of firing. There is no danger connected with the shooting of cartridges having split necks, but their continued use is likely to cause some erosion of the chamber neck which may lead to extraction difficulties. If such ammunition is used, a loaded cartridge should never be extracted from the chambcr without making sure that the bullet does not remain in the barrel. Should the bullet remain in the barrel, it might be possible to seat another cartridge having a loose bullet behind the first one. Forcing the second bullet back onto the powder charge would increase the loading density and cayse a rise in pressure, to say nothing of the great increase in pressure that would result from the weight and resistance offered by two bullets. Under such circumstances, if the arm were discharged, the barrel would be ruined and the pressure might be sufficient to causc the cartridge case to give way at the head, wrecking rifle. The usual effect of firing two bullets at the same time is to ring or bulge the barrel. The rear bullet starts out at a greater velocity than the forward one. The resistance offered by the forward bullet causes the point of the last and the base of the first bullet to expand violently at some point along the barrel and the radial pressure is sufficient to ex-pand the steel outward. This makes a visible, dark ring in the barrel and may cause a bulge on the outside. Occasionally a barrel will split lengthwise irom this cause. Lead bullets can make just as nice rings in a barrel as jacketed bullets, and high pressures or velocities are not necessary to do it either. The lowly .22 rim fire cartridge can ring a barrel beautifully, if two bullets are fired at one time. Incidentally, ringing a barrel in this way will not affect its accuracy, at least, this writer has ringed a few barrels and has shot quite a few others that were ringed and has never observed any loss in accuracy from this cause. But, as far as cartridges with split necks are concerned, by far the best policy is to not shoot such cartridges. The bullets and the powder from them can be salvaged and loaded into good cases with very little effort and expense»

Season Cracking. This condition arises from internal stresses in the brass itself, if brass is too hard it may crack spontaneously in time, especially in hot climates or if subjected to corrosive gasses, also long continued strain of any kind may cause it. Season cracking is not a condition that occurs only in cartridge cases but is more or less common to all drawn brass articles. The condition is most frequendy encountered in the form of split necks in .30-06 ammunition manufactured during the World War. This ammunition was made hurriedly and with the belief that it would be used within a relatively short period of time. The necks of most of the cases were quite hard and under the strain imposed on them by holding the bullets under tension, the necks were apt to crack after a time. Since die war, the necks of practically all cases of rifle ammunition have been subjected to an additional annealing process that relieves the internal stresses, without rendering the brass too soft to hold the bullets properly. This makes the case necks better able to withstand the repeated reducing and expanding that is often necessary to properly reload them. Season cracking in small arms ammunition is practically a thing of the past and is only mentioned here as being of casual interest.

Body of the Case. Practically everything that has been said above with regard to the expansion of the necks of cartridge cases applies to the expansion of the body of the case as well. If the case is of the straight or cylindrical type, the body and neck arc continuous and in this type the term "neck" is applied to that part of the case that is normally occupied by the bullet In a straight taper case, such as the .32-40, .38-55, 45-90, etc., there is no definite line of demarkation between the cylindrical ncck that holds the bullet and the tapered body. The distinction is very definite in the bottle-neck ease, but in referring to the 1 body" we mean that part of the case between the neck or shoulder and the head, or the part of the case that contains the powder charge.

The degree of expansion of the body usually determines whether the case must be completely resized before reloading or not. The resistance offered by an cxccssivcly expanded neck is not of itself sufficient to offer difficulty in extracting the case from the chamber. All cases are not of the same degree of hardness or "springiness." The product of any one manufacturer will be found to run quite uniform, but there is often a considerable difference in the cases of dif ferent makes. Soft cases will not spring back from the chamber walls quite as much as larder cases and if soft cases arc reloaded with full charges they are apt, after one or more reloadings, to be not only difficult to extract but also difficult to re-enter in the chamber in which they were fired. The remedy is to resize them in a suitable die and in doing this it is a good rule to only resize them enough to permit their entering the chamber easily. The complete resizing of cases should be avoided wherever possible, for reasons that will be explained later.

Splits or ruptures may occur in the bodies of cartridge cases; splits from excessive expansion, hard brass, defects in the brass or a combination of these things. Ruptures in the bodies of cases (except near the heads) are usually due to manufacturing defects and are of rare occurrence. Neither of these defects will cause injury to the arm or the shooter, provided that the break docs not occur dose to the head. The body of the case is, "just the part in between." The relation of the case neck to the neck of the chambcr is of 21 importance co accuracy as will be explained later, while the head, and the body for a short distance in front of it, takes much of the stress of firing and is important from a safety standpoint. The body just connects these two parts together and no harm is likely to occur if it splits.

Micro-photographs of brass—showing difference in erystaDtne structure.

A Set of Machine Gun Chambering Reamers.

Was this article helpful?

0 0

Post a comment