Immediately after leaving the barrel, the bullet is in a slightly unstable condition which is due, in the main, to three factors: 'yaw', ' precession' and 'nutation'.
Yaw. Yaw can be described as the angle between the longitudinal axis of a projectile and its line of flight as exists before the bullet achieves full gyroscopic stability (Figure 3.7).
Precession. This is the rotational effect of the bullet about its mid axis (Figure
Nutation. This is the progressive corkscrew motion of the bullet. This action is very similar to the wobble observed immediately after a top or gyroscope is initially set spinning and is a function of the spin rate being too great (Figure 3.9).
As in a top, these factors eventually settle down to a stable flight pattern which in rifles can be anything up to 200 yd. It is this initial instability that often accounts for the far greater close range wounding effects of hard-jacketed rifle bullets when compared to those at greater distances.
There is a fourth condition which imparts a sideways drift to the bullet's path called either 'spin drift' or 'gyroscopic drift'. This is to the right for right-handed rifling and to the left for left- handed. It is caused by air pressure under the slightly nose up attitude of the bullet as it descends its trajectory. This effect is really only noticeable on extreme range rifle bullets or artillery shells.
Figure 3.7 Bullet yaw.
Figure 3.8 Precession of a bullet.
Figure 3.9 Nutation of a bullet.
Angles of yaw have only received detailed examination in military weapons, for example, the measured angle of yaw for a 0.303" rifle bullet is 1.5 ° and for a 5.56 mm M16 rifle bullet 6 °.
This yaw does have a pronounced effect on the wounding capabilities of the missile. The greater the degree of yaw, the greater the wounding effect of the bullet. This yaw effect also explains the commonly observed effect of a rifle bullet having greater penetrative powers at 200 yd than at the muzzle.
Case example. For a long time, I had been aware of the instability of rifle bullets at short ranges and their apparent inaccuracy due to a changing point of impact resulting from nutation and precession. However, it was not until I was asked to explain some external and terminal ballistics effects to camera that the extent of this instability was fully realized.
The programme was designed to explain the capabilities of various historical and current military weapons and the wounding effects they would have on a human body. To visualize the temporary cavitational effect of the bullets, the weapons were fired at large blocks of modelling clay. This was carried out at a range of 25 m.
As a result of the close range instability factor of the 0.303" Enfield bullet, I extended the range of firing for this weapon to 250 yd.
As expected, the bullet made a small hole straight through the modelling clay with little in the way of any temporary cavitational effect.
The director was of the opinion that firing at such a range was not good TV footage and insisted that the block be shot again at 25 m. This time, the effect of the bullet on the 2 x 2 x 4 ft block of modelling clay was catastrophic with it literally exploding into small pieces. On gathering up the pieces, all that could be found of the bullet was a very small piece of jacket and two equally small pieces of lead core.
It was obvious that the instability due to the nutation and precession of the bullet had been greatly increased when the bullet entered the clay. The resulting G forces on the bullet had literally torn it apart releasing its kinetic energy in a very short period of time. The dumping of such a huge amount of energy in a very short period of time had caused an almost explosive effect on the clay.
Despite it not being representative of the battle range wounding capabilities of the 0.303" round, the footage was included as part of the series.
The rate of spin imparted to a bullet by the barrel 's rifling is calculated to ensure that the bullet is stable in air. Once it enters a denser medium, however, the spin is insufficient to stabilize the bullet and it begins to wobble. As this exposes a greater cross- sectional area of the bullet to the tissue, the wobble becomes greater until eventually, the bullet begins to tumble end over end.
As the tumbling bullet exposes a much greater area of the bullet, the wound track and the kinetic energy loss will be tremendously increased.
In lightly jacketed bullets, this tumbling can also cause the bullet to break up causing a massive increase in the kinetic energy loss and a consequential increase in the temporary and permanent wound cavities. This effect is much greater in short projectiles, such as the 0.223" (5.56 mm) Ml6 bullet, than with longer bullets such as the 0.303" British military round.
The wounding effect of a missile is also dependent on the shape of the bullet nose. A round-nosed bullet will be retarded more than a sharp-pointed bullet. An expanding, hollow-point bullet will be retarded even more.
The amount a bullet will deform during passage through soft tissue will depend very much on the construction of the bullet. A fully jacketed bullet will hardly deform at all, whilst a soft- lead hollow- point bullet will deform very easily.
For a hollow-point soft-nosed bullet to expand, a velocity of at least 900 ft/s is required. A round-nosed plain lead bullet will require at least 1200 ft/s.
Numerous reports exist to the effect that the 5.56 mm (0.223") M16 bullet 'blows up' on striking soft tissue. This is nonsense. What does happen, however, is that the thinly jacketed bullet, which is only just stable in air, becomes very unstable in tissue and starts to tumble. This tumbling action presents a much larger surface area of the bullet to the retarding tissue. This increased surface area causes tremendous strain on the bullet's structure which results in the jacket rupturing and the lead core fragmenting.
With an initial bullet velocity of over 3000 ft/s, the kinetic energy loss is already tremendous, but as the bullet breaks up, it becomes even greater. Massive permanent cavities, huge temporary cavities and tremendous damage to organs, blood vessels and bone remote from the wound track give rise to an appearance which many associate with the result of the bullet exploding.
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