For example, the muzzle velocity of any given cartridge» as printed on the cartridge box or in the ballistic tables of the manufacturer, cannot be used as the basis for any calculations bccausc these figures cannot be depended upon to be correct. This is due largely to changes that are necessary in the loading of the ammunition when changing from one lot of powder to another. The same thing holds true of pressures and such pressure figures as arc published in tables of charges based upon the results obtained with one load of cartridges in one gun.
As a concrete example of the variations in the performance of different loads of ammunition we can consider the cal. .30-06 Mr cartridge, which is the present standard ride and machine gun cartridge in the United States army. The standard average instrumental velocity of this cartridge is prescribed in specifications as 2640 f.s., plus or minus a small tolerance. For tactical reasons this velocity must be maintained from one lot of ammunition to another to a much higher degree of uniformity than is necessary in any commcrcial sporting cartridge. Yet every experienced rifleman knows that different lots of this ammunition may require a considerable difference in sight settings, especially at the longer ranges. Therefore, the handloader should judge his ammunition from the view point of actual performance, rather than by fooling around with any mechanical computations.
Another example which will show that mechanical computations give only an approximate result, can be found in the firing of seacoast cannon. The range finding and load data for every shot fired out of one of these big guns is a matter of permanent record. Furthermore, there is provision for fastening two pressure gauges in the mushroom heads of the breech blocks of these guns so that the actual chamber pressures developed can be measured, and the powder charges adjusted properly before commencing any 262 range firing. In spite of these complete records and the duplication of charges that have been fired in the gun before, it is not uncommon for the first shots fired at a target to be as much as a thousand yards or more over or short of the target, necessitating an arbitrary correction in order to place the shots in the vicinity of the target—and this without any error on the part of the range or loading details, Therefore, we are going to be very practical in treating the subject of ballistics and leave the theoretical aspects to the text books, where they properly belong.
The subject of ballistics is divided into two parts: interior ballistics, which concerns the time between which the trigg^ a firearm is pulled and the bullet is far enough out of the muzzle of the gun so that it is no longer affected by the expanding gasses; and exterior ballistics, which has to do with the free movement of the bullet or projectile through the atmosphere.
Interior ballistics, as related to small arms, is divided into four distinct phases. The first is the interval between the time the s^ar is released and the firing pin impresses itself into the primer sufficiendy to promote ignition of the latter. The second phase represents the time required for the primer to transmit its flash to the powder chargc and ignite a sufficient amount of it to promote combustion of the remainder. The third is the interval between the time the powder begins to burn, and transform into gasses, and the time that the bullet starts to move forward. The fourth and last phase is the time between the initial movement of the bullet and the time that the bullet is out of the muzzle and beyond the effect of the expanding powder gasses behind it. With proper facilities, these several time elements can be calculated or estimated, it being necessary to measure such intervals in units of one ten-thousandth of a second—a time measurement that is too small for human conception. However, the time of each of these phases will differ considerably in different cartridges, and will change with any change in the components of the loading^ of any cartridge. Therefore, there is no use in even attempting to suggest what these time intervals might be in handloaded cartridges.
When a primer is struck by a firing pin with sufficient force to indent it appreciably, the priming pellet is pinchcd against the anvil and a flame is produced which passes through the vent or flash hole (sometimes more than one) in the bottom of the primer pocket and into the body of the cartridge case, where the propelling charge is located. This flame normally ignites more or less of the charge, which begins to burn at atmospheric pressure just as powder burns in the open air when ignited with a match. As the first grains of powder begin to burn, gas is given off which can find no escape from the dosed chamber and as the volume of gas continues to increase, the pressure rises. This first evolution of gas is known as "new" or "young" gas and as it devdops, and the pressure rises, the walls of the cartridge case are expanded and pressed against the walls of the chamber—the neck of the case sharing in this expansion. The bullet has not yet begun to move and, with the neck of the case expanded, the new gas rushes past the bullet, escaping out of the barrel ahead of it.
nutlets which skidded. A couple or bullets that never did get rotated properly by the rlfltiijj. Too soft and ton short tor the powder charges used.
The bullet on the lert was fired through a barrel with deep truoves. The one on the right through an oversize barrel. Note how the gas, rushing past both, has carried the edg«6 of the bands away or rounded them.
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