What is Ballistics?

 

INTERIOR BALLISTICS

Interior ballistics covers the events inside a gun that occur from the time the firing pin strikes the primer until the bullet exits the muzzle. While the internal ballistics of large cartridges such as cannons are reliably predictable from mathematical considerations, interior ballistics for small arms are difficult to predict at least by reasonable calculation procedures. In the past, ballistic data for small arms have been found primarily by velocity and pressure tests or empirical methods rather than by calculation. Many firing tests have been performed by several powder and bullet manufacturers for a large number of bullet/powder combinations, in small arms. Because of these tests, sufficient data has emerged that when coupled with sound ballistic correlations have allowed the calculation of small arm ballistics with reasonable accuracy. Such pioneers as Homer S. Powley have added much to the understanding of small arm interior ballistics and his POWLEY COMPUTER FOR HANDLOADERS has been a real help to dedicated handloaders over the years.

The familiar kinetic energy equation (E = 1/2mv2) when equated to the energy available from the powder charge driving the bullet down the gun barrel will allow the calculation of bullet velocity with reasonable accuracy.

A pound of single-base rifle powder has an energy content of about 1,246,000 ft-lbs of energy or about 178 ft-lbs per grain of powder. In actual practice only a fraction of this energy is available to accelerate the bullet. Julian Hatcher in HATCHER'S NOTEBOOK reported the energy distribution for the Browning Machine Rifle as follows:

Heat to cartridge case		4%
Kinetic energy to bullet		29%
Kinetic energy to gases		19%
Heat to barrel		22%
Heat to gases		19%
Heat to bullet friction		7%
		100%

You will note that the energy imparted to the bullet is only about 29% of the total powder energy available. While this is typical of many small arms cartridges, actual efficiencies may range from 17 to 37 percent or more. The actual efficiency is basically a function of expansion ratio and charge weight to bullet weight ratio.

For optimum cartridge performance it is necessary to burn as much powder as the case will comfortably hold while developing a pressure that the rifle can comfortably stand. This criteria is maintained by using a loading density of 80 to 90 percent and selecting a powder type that, in most instances, produces a peak chamber pressure of around 40,000 to 50,000 psi. This pressure limitation is generally safe for modern rifles in good condition yet assures good barrel life while promoting complete powder combustion and high efficiency. Remember that the pressure calculated is only an estimated value. For safety sake --- always use the starting load suggested by the program or loading manual data and work up to the calculated powder charge in small increments, watching for signs of excess pressure along the way. Stop immediately if you encounter hard or sticky case extraction, excessive primer flattening or case head burnishing upon extraction.

Several factors enter into the proper powder selection for a given cartridge/bullet combination. For example, the burn rate of powder (sometimes referred to as relative quickness) is governed to a great extent by pressure developed within the cartridge. For this program the working pressure has been established at 40,000 to 50,000 psi, requiring that the powder burning rate (relative quickness) be matched to bullet acceleration to produce optimum velocity. For a given pressure the optimum velocity can only be obtained by maintaining the accelerating force for a longer period of time. Fortunately, the relative quickness of IMR and similar single base powders can be related to the bullet sectional density and the powder to bullet mass ratio for most cartridges. These relationships were adopted to a series of equations for proper powder selection and included in this program. The computer selection includes IMR, Hodgdon, Hercules, Accurate Arms, Scot, Winchester, and other powders. For those of you who may have discontinued powders such as Hercules RX11 and RX21, these are included also.

Recoil energy is calculated in this program using a method suggested by W. C. Davis, Jr. in the July, 1980 issue of the American Rifleman magazine. Generally, this method gives somewhat lower recoil values than the method suggested by Mr. Julian Hatcher, page 290 of Hatcher's Notebook, but seem to be in better agreement with experimental data.

Pressures for this program are calculated using a modified pressure equation suggested by Homer Powley. For the alternate loading table, velocity is assumed to vary directly as the ratio of the powder charge and pressure as the square of the ratio of the powder charge.

It should be remembered that the computed velocities and pressures are only estimates and will vary from rifle to rifle. Such variables as primer type, freebore travel, bullet friction and case shoulder angle all have an effect on developed pressure and resultant velocity. These vary for different rifles. However, I believe you will be pleasantly surprised how well the predicted velocities approach actual measured velocities for most cartridges.

 

EXTERIOR BALLISTICS

While interior ballistics deals with events inside a gun, exterior ballistics covers those events that occur from the time the bullet leaves the muzzle until it strikes the ground downrange. When a bullet leaves the gun, it contains kinetic energy which tries to move it in a straight line with its initial velocity. However, gravity pulls the bullet toward the ground, and air resistance tries to impede it. The result of these forces cause the bullet to follow a drooping curve called a trajectory and it soon strikes the earth downrange. The air resistance forms a retardation along the bullet's trajectory path.

Near the turn of the century, many tests were made by the Krupp Company of Germany to determine the retardation or drop characteristics of so-called standard bullets. Soon after the Krupp data were published a Russian army colonel named Mayevski constructed a mathematical model for the drag deceleration of a standard bullet. Colonel James M. Ingalls of the U.S. army later used Mayevski's mathematical model to compute his now famous ballistics table.

Today, most of the major bullet companies use the Ingalls or similar tables together with test firings of production bullets to compute ballistic coefficients for their bullets. These coefficients are published in most of the major reloading manuals that are on the market today. Ballistic coefficients, from American manufacturers, are included in the Load From a Disk database.

The ballistic data used in this program are based on a least squares curve fit of time and space functions as they appear in the Ingall's tables. Values generated by these equations produce results that are in close agreement with with the original data. Ballistics for "point blank range" calculations are based on a series of correlations developed and published by Dr. Ralph McGehee in the early 70's.

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