but their general course will represent a decided flattening of the pressure curve all out of proportion to the flattening of the velocity curve. The higher pressures go, the smaller the increase in velocity for each increase in the charge. In the pressure curve there will be a range where the plotted points will coincide quite well with the smoothed out curse. This uniform range represents the tolerance of the powder and the point of the greatest uniformity is the balance point. The tolerance may be wide for one powder and narrow for another, but that is the way the job is done. In making up tables of charges, the powder companies must establish curves, not only for cach powder but for every cartridge that those powders are used in and for each different bullet as well. It can be seen that there is an immense amount of work, to say nothing ot expense, in back of every one of these tables. We handloaders are indeed fortunate in having such authentic and complete data available for the asking.
In the acceptance tests of military powders, the charge, pressure and velocity curve must show that the powder being tested will develop the proper velocity in the cartridge and with the projectile specified, without exceeding the permissible pressure. The velocity that the projectile must have and the maximum permissible pressure for the guns the powder is to be fired in, are iixed in the specifications and the powder must meet those specifications or it will be rejected. This is necessary because the military requirements of the ammunition are such as to necessitate a uniform muzzle velocity from one lot of ammunition to another.
This need for rigid powder specifications does not exist in commercial sporting ammunition except in a few special instances. Consider our own military requirements for powders for small arms ammunition; we have only three calibers, .30, .45 and .50 and we could get along with only three different powders if we had to. Now in comparison, just take a look at the variety of cartridges, different weights of bullets with which they arc loaded, etc., in any ammunition catalogue. Each one of those cartridges presents its own $7 loading problems and the range is so great that the commercial manufacturer needs a variety of powders of widely different burning characteristics. Even though a new lot of powder may not bum properly in the range of cartridges it was manufactured tor, its burning characteristics may be exceptionally good when fired in a different range of cartridges than that for which it was intended. For example, a powder made for the .30-06 cartridge may, on test, be found to develop too high pressures, which indicates that it burns too quickly for the .30-06 and that when tested in the Cal. .250-3000 cartridge it gives high velocities with per-fecdy safe pressures. The commercial manufacturer has all the necessary laboratory equipment for determining the suitability of powders for his loading requirements and can use lots of powders that are 4,off" from the standard, very nicely. For this reason there is no certainty as to the kind of powder that is used in different lots of any caliber of commercial cartridge and commercial ammunition is subject to wider variations in velocity from one lot to another than military ammunition. The manufacturer endeavors to load as closely as possible to the advertised velocity but the primary objects sought arc satisfactory performance and safety.
Here in the United States pressures arc customarily measured by what is known as the radial system. A heavy steel yoke is mounted around and over the chamber of the barrel in which the pressures are to be taken. A hole is drilled through the base of the yoke that encircles the barrel, into the chamber. A steel piston is closely fitted to the hole, with just enough clearance so it may move up and down freely. A heavy thumb screw passes down through the top of the yoke and a steel block, calicd the "anvil", is made to fit loosely inside of the yoke. The fit of the anvil is such that it can be readily removed and replaced and its upper and lower surfaces are ground smooth and parallel. This constitutes the mechanical part of the gage. Pressure gages for rifle cartridges usually have the piston located one inch from the head of the cartridge but for revolver and pistol cartridges the pistons arc located just ahead of the front edge of the cartridge ease.
A small copper cylinder called a "crusher" is used to measure the pressures developed within the chamber. These crushers must be of uniform hardness, or more properly, softness, as it is upon their uniformity of viscosity that the uniformity and accuracy of the readings depend. The method of using a crusher gage is as follows. A cartridge is placed in the chamber and a gas check cup, similar to a gas check used on a cast bullet or a primer without an anvil, is filled with grease and inserted in the piston hole, base up. The piston is inserted on top of this and pressed down until the edge of the gas chcck is in contact widi the cartridge. The gas check and grease serve to prevent the escape of gas past the piston. A crusher is placed on it, first having been carefully cut to length and measured with a micrometer caliper, end on top of the piston and the anvil on top of the crusher. The thumb screw is turned down so as to bear firmly on the anvil, but not with a pressure that will disturb the dimensions of the crusher. This thumb screw, through the anvil, supports the thrust of the crusher when the gun is fired. When the cartridge is fired, the internal pressure, acting radially against the chamber walls, also acts upon the piston of the gage, forcing it upward and compressing the crusher. The crusher is then removed and measured again to determine the reduction in its length and the amount of reduction is used as a measure of the maximum pressure developed in the chamber. The pressure is expressed in terms of pounds per square inch, but actually it is nothing of the kind. This method is crude and has many faults but it is convenient and affords sufficient accuracy to insure the loading of safe ammunition. The figures obtained vary considerably, for many reasons that need not be explained here and arc subject to interpretation by those whose experience makes them competent to do so. The average person is apt to consider the numerical values of pressures, expressed in terms of pounds per square inch in powder chargc tables, as fixed values of measurement in the sense that a one pound weight or a foot rule are fixed values. This is a mistake and other than to indicate a maximum point beyond which charges should not be used, they are of no particular value in tables of charges.
One thing a crusher gage does not show is the time required for the pressure to reach its maximum point. If the pressure is close to the bursting point of a gun, this element of time is of great importance for if the maximum point is reached too quickly, the molcculcs of the steel will not have time to adjust themselves to the strain and the gun will burst. As an example of the effect of the element of time on applied force, a simple experiment can be made with a piece of ordinary tar. At room temperature the tar is quite hard, but it may be slowly bent or deformed with the hands. All that is necessary is to apply the force slowly enough so that the molecules can adjust themselves to it. But if we strike the tar a sharp quick blow it will break, because the force is applied so quickly that there is insufficient time for the molcculcs to readjust themselves, although the energy of 33 the blow may be even less than that previously applied with the hands. It doesn't pay to experiment blindly with powder charges when the pressures are around the upper limits of the tolerance, nor is it necessary to do so. Both the duPont and Hercules powder companies are willing to assist re-loaders who desire to experiment with unknown loads and will make pressure determinations for them at a very reasonable charge; about one dollar per shot, if I remember cor-recdy. This may seem like a lot of money, but the reader should bear in mind that it costs around two hundred dollars to make a pressure gage and the life of one of these gages is only from about two hundred or less shots up to five hundred at the most. From this it will be seen that the charge made hardly more than pays for the wear and tear oa the gage.
The speedometer of an automobile, as its name indicates, registers the speed in miles or kilometers per hour that the car is traveling at, at any particular time. If we wish to determine the average speed of the car between any two points, a speedometer is useless and we must use a time piece, taking the time cf departure, the time of arrival and dividing the elapsed time by the number of miles traveled; for under ordinary driving conditions, it is impossible to drive a car at a uniform rate of speed.
Bullets have no speedometers on them, nor do they travel at a fixed rate of speed and their velocities must be measured with an instrument that measures time; the time required for the bullet to travel over a known distance. To measure the velocities of bullets and projectiles, instruments known as chronographs arc used, dieir name signifying the graphic measurement of time.
The instrument most used for this purpose is the Boulcnge chronograph, the invention of a Belgian army officer whose name it bears. This chronograph has been modified in several ways during the many years it has been used but the fundamental principle is still retained, viz.; the measurement of elapsed time through the medium of 9 two falling weights. The design and operation of this instrument can be understood from die accompanying simplified diagram.
The instrument consists of a solid base mounted on legs that can be adjusted to level it and set on a solid bench. The base supports a substantial vertical column about three feet high to which are attached two electro-magnets, one higher than the other. The circuits for these two magnets are independent of one another but a means is provided interrupting both circuits at the same instant, in order to obtain a zero point. This device breaking both circuits at once is calicd the "disjunctor." Their cores are, of course, only magnetized while the electric current is passing through them and must be uniform as to magnetic "lag,"
or die retention of magnetism, after the circuit is broken. 91 A long, steel rod called the "chronometer" is suspended from the high magnet and a short one called the "registrar" from the low magnet. Both rods have soft iron tips that demagnetize quickly. A copper or zinc tube is slipped over the chronometer rod and extends nearly the length of the rod. This tube is called the "recorder" and, when in place, the weights of the two rods arc the same, or about one pound each.
On die base is a knife actuated by a spring and so located that the chronometer, in falling, passes close to its edge when the knife is cocked. The knife is held in the cocked position by a flat, plate-like trigger which extends under the registrar.
The current is usually supplied by storage batteries and passes through rheostats and ammeters connected in the circuits of each of the clectro-magncts, By means of the rheostats and ammeters, the strength of the currcnt and consequendy of the magnetic fields can be equalized in both magnets, so that each will lose its magnetism at the same speed when the circuit is broken.
While the circuits through the two magnets are independent of one another, they both pass through the dis-junctor, whereby both circuits may be broken at the same instant. The independent circuit of the magnet supporting the chronometer rod passes through a fine wire, called the muzzle wire, strctchcd across the path of the bullet and located close to the muzzle of the gun. The circuit for the registrar magnet is completed through a circuit interrupter placed at some distance from the muzzle of the gun. For measuring the velocities of small arms bullets, this interrupter usually takes the form of a piece of armor plate with a hardened surface that will not be deformed by the repeated impact of bullets, having a delicate adjustable spring contact on its back. The adjustment of this spring contact is so dclicatc that any jar of the plate will cause the circuit to break. The spring causes it to remake contact immediately, which eliminates any need of manual operation and saves time. In measuring the velocities of artillery pro- 92 jectiles, two wire screens are used, the projectiles breaking the wires as they pass through. Such screens must be re paired or replaced after each shot and in small arms work are not often used except for armor piercing bullets.
After die chronograph is adjusted and before doing any firing, the chronograph and registrar rods arc "hung up" on their respective magnets and both circuits are broken at the same instant by means of the disjunctor, so that both rods will fall at the same time. When the registrar strikes the trigger, the knife is released and flies out, making a cut or nick on the recorder or tube carried by the chronometer rod at the point which is opposite the knife at that instant. The distance that the chronometer drops before the knife strikes it represents the free fall and will be constant from one shot to another. The mark made on the recorder is the 93 zero mark, or disjunctor point, from which subsequent measurements are taken.
The rods are then hung up again and a shot fired. The instant the bullet breaks the muzzle wire, the circuit in the chronometer magnet is broken and the chronometer rod begins to fall; and when the bullet strikes the terminal target, breaking the circuit in the registrar magnet, then the registrar falls. Both rods continue to drop together but when the registrar strikes the trigger, the knife flics out and cuts a nick in the recorder at the point opposite the knife edge at that instant. The distance between the zero mark and the one made by firing represents the time required for the bullet to pass from the muzzle wire to the terminal target and as this distance is definitely known, it is a simple matter to calculate the time of flight and the velocity in feet per second. As a matter of fact, such calculation is not necessary as the scale with which the distance between the marks is measured is graduated to read direcdy in feet per sccond, thus saving a lot of rime and trouble.
In taking rifle velocities, the muzzle wire is located three feet in front of the muzzle to avoid its being broken by the muzzle blast. The terminal target is located 150 feet from the muzzle wire and this distance, plus the distance from the muzzle of the rifle to the muzzle wire makes a total of 153 feet that the bullet travels, although the velocity is measured only over 150 feet. In this case the velocity obtained is the average over the 150 feet between muzzle wire and terminal target, and is the velocity at the midpoint of this distance, or at 78 feet from the muzzle. Sometimes the terminal target is placed 100 feet in front of the muzzle wire, giving a velocity at 53 feet from the muzzle; or any other convenient distance may be used.
For pistol or artillery velocities, the distances are smaller and greater respectively and each measuring scale is stamped with the distance between "screens" that it is suited for.
Velocities taken as described are the average velocities over the distance and are called "instrumental velocity.'' As a bullet begins to slow up once it is beyond the influence of the muzzle blast, it is not traveling as fast when it strikes the target as when it left the muzzle of the arm it was fired from. It is a difficult matter to find the exact velocity of a bullet at the instant it leaves the muzzle of a rifle, but it can be found approximately as follows:—Mark one horizontal edge of a sheet of cross section paper off in units of feet, starting with zero at the right to represent the muzzle
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