More powder can also be used in rifle cartridges because the bullet chambers can be designed to withstand greater pressures 50, to 70, for rifles psi vs. Higher pressures require a bigger gun with more recoil that is slower to load and generates more heat that produces more wear on the metal. It is difficult in practice to measure the forces within a gun barrel, but the one easily measured parameter is the velocity with which the bullet exits the barrel muzzle velocity and this measurement will be used in examples below.
Bruner et al, The area here is the base of the bullet equivalent to diameter of barrel and is a constant. Therefore, the energy transmitted to the bullet with a given mass will depend upon mass times force times the time interval over which the force is applied. The last of these factors is a function of barrel length.
Bullet travel through a gun barrel is characterized by increasing acceleration as the expanding gases push on it, but decreasing pressure in the barrel as the gas expands.
Up to a point of diminishing pressure, the longer the barrel, the greater the acceleration of the bullet. Volgas, Stannard and Alonso, As the bullet traverses the barrel of the gun, some minor deformation occurs, called setback deformation. This results from minor rarely major imperfections or variations in rifling or tool marks within the barrel.
The effect upon the subsequent flight path of the bullet is usually insignificant. Jandial et al, The external ballistics of a bullet's path can be determined by several formulae, the simplest of which is:. This is the bullet's energy as it leaves the muzzle, but the ballistic coefficient BC will determine the amount of KE delivered to the target as air resistance is encountered.
Thus, greater velocity, greater caliber, or denser tissue gives more drag. The degree to which a bullet is slowed by drag is called retardation r given by the formula:. SD is the sectional density of the bullet, and I is a form factor for the bullet shape. Sectional density is calculated from the bullet mass M divided by the square of its diameter. The form factor value I decreases with increasing pointedness of the bullet a sphere would have the highest I value.
Since drag D is a function of velocity, it can be seen that for a bullet of a given mass M , the greater the velocity, the greater the retardation. Drag is also influenced by bullet spin. The faster the spin, the less likely a bullet will "yaw" or turn sideways and tumble in its flight path through the air.
Thus, increasing the twist of the rifling from 1 in 7 will impart greater spin than the typical 1 in 12 spiral one turn in 12 inches of barrel. Bullets do not typically follow a straight line to the target. Rotational forces are in effect that keep the bullet off a straight axis of flight. These rotational effects are diagrammed below:.
Yaw refers to the rotation of the nose of the bullet away from the line of flight. Precession refers to rotation of the bullet around the center of mass. Nutation refers to small circular movement at the bullet tip. Yaw and precession decrease as the distance of the bullet from the barrel increases. What do all these formulae mean in terms of designing cartridges and bullets? Well, given that a cartridge can be only so large to fit in a chamber, and given that the steel of the chamber can handle only so much pressure from increasing the amount of gunpowder, the kinetic energy for any given weapon is increased more easily by increasing bullet mass.
Though the square of the velocity would increase KE much more, it is practically very difficult to increase velocity, which is dependent upon the amount of gunpowder burned. There is only so much gunpowder that can burned efficiently in a cartridge. Thus, cartridges designed for hunting big game animals use very large bullets.
To reduce air resistance, the ideal bullet would be a long, heavy needle, but such a projectile would go right through the target without dispersing much of its energy. Light spheres would be retarded the greatest within tissues and release more energy, but might not even get to the target.
A good aerodynamic compromise bullet shape is a parbolic curve with low frontal area and wind-splitting shape. The best bullet composition is lead Pb which is of high density and is cheap to obtain. Alloying the lead Pb with a small amount of antimony Sb helps, but the real answer is to interface the lead bullet with the hard steel barrel through another metal soft enough to seal the bullet in the barrel but of high melting point.
Copper Cu works best as this "jacket" material for lead. Yaw has a lot to do with the injury pattern of a bullet on the target, termed "terminal ballistics. This causes more tissue to be displaced, increases drag, and imparts more of the KE to the target. A longer, heavier bullet might have more KE at a longer range when it hits the target, but it may penetrate so well that it exits the target with much of its KE remaining.
Even a bullet with a low KE can impart significant tissue damage if it can be designed to give up all of the KE into the target, and the target is at short range as with handguns. Despite yaw, an intact bullet that comes to rest in tissue generally has its long axis aligned along the path of the bullet track, though its final position may be either nose forward or base forward. Laceration and crushing - Tissue damage through laceration and crushing occurs along the path or "track" through the body that a projectile, or its fragments, may produce.
The diameter of the crush injury in tissue is the diameter of the bullet or fragment, up to the long axis. Cavitation - A "permanent" cavity is caused by the path track of the bullet itself with crushing of tissue, whereas a "temporary" cavity is formed by radial stretching around the bullet track from continued acceleration of the medium air or tissue in the wake of the bullet, causing the wound cavity to be stretched outward.
For projectiles traveling at low velocity the permanent and temporary cavities are nearly the same, but at high velocity and with bullet yaw the temporary cavity becomes larger Maiden, Shock waves - Shock waves compress the medium and travel ahead of the bullet, as well as to the sides, but these waves last only a few microseconds and do not cause profound destruction at low velocity.
At high velocity, generated shock waves can reach up to atmospheres of pressure. DiMaio and Zumwalt, However, bone fracture from cavitation is an extremely rare event. Fackler, The ballistic pressure wave from distant bullet impact can induce a concussive-like effect in humans, causing acute neurological symptoms. Courtney and Courtney, The mathematics of wound ballistics, in reference to yaw of unstable projectiles, has been described.
The model works well for non-deformable bullets. Peters et al, Peters and Sebourn, Experimental methods to demonstrate tissue damage have utilized materials with characteristics similar to human soft tissues and skin.
Pigskin has been employed to provide an external layer to blocks of compounds such as ordnance gelatin or ballistic soap. Firing of bullets into these materials at various ranges is followed by direct visual inspection cutting the block or radiographic analysis CT imaging to determine the sizes and appearances of the cavity produced Rutty, et al, The following images illustrate bullet deformation and damage:.
Bullet velocity and mass will affect the nature of wounding. Wilson, An M rifle. A hunting rifle. Bullet design is important in wounding potential.
The Hague Convention of and subsequently the Geneva Convention forbade the use of expanding, deformable bullets in wartime. Therefore, military bullets have a full metal jacket FMJ around the lead core. Such bullets are typically made of a metal other than lead, such as copper powder compacted into a bullet shape, as diagrammed below:.
The distance of the target from the muzzle plays a large role in wounding capacity, for most bullets fired from handguns have lost significant kinetic energy KE at yards, while high-velocity military. But this shock wave is not directional.
Generating this shock wave requires a relatively high amount of energy. From shooter to target the bullet is dropping after it noses over at apogee, the highest point of the bullet arc. We fire upwards at an angle to compensate for bullet drop, this arc stretches out from the muzzle forming an upward angle of departure.
If our gun is sighted in at a hundred yard there will be zero drop, at the yard mark. Let us do some ballistic comparisons. To pick a standard we will use the 7. It takes a skilled shooter to consistently make shots beyond yards. An ethical hunter clearly understands his or her capability and refrains from attempting shots that are beyond the effective range of the hunter and the firearm. Various scopes, laser range finders, and angle-compensating devices all help in making the mid- to long-range one shot, one kill a reality.
A simple tool to aid the hunter is to tape a small drop chart Figure5 somewhere on your rifle. A good drop chart will detail the range of each mil dot or elevation graduation of your rifle scope. A commonly accepted threshold for the minimum amount of kinetic energy needed to kill an elk is ft-lbs.
For whitetail deer, the minimum amount of kinetic energy is ft-lbs. But is this really the most ethical caliber to use? If the elk was standing broadside at yards, the. Remington grain Core Lokt PSP would only have the minimum energy ft-lb to take down a whitetail. The elk could run off wounded and be left to suffer. Colorado elk are often found in very rugged terrain. A mortally wounded elk can still go over a 10,foot ridgeline that it may take the hunter a solid hour to get over on foot.
A lightly wounded elk can cover the mile or two between you and the next valley in a couple of minutes. Another complication is the access in elk country may not be the same as in whitetail country. A big part of being an ethical hunter means treating the elk with respect.
One way to do that is by making a quick clean kill. A downed elk is significantly more work cleaning and quartering than a whitetail deer. Rather than betting on a. At yards, a. For example, the. Is more always better? Ultra-magnum rifles do have a lot of power, but they also have a lot of felt recoil.
0コメント