Bullet selection for hunters -II

LUTZ MOELLER

Picture: NATO M855/SS109 5,6 x 45mm  Full metal jacket bullet fragments depending on speed more ore less

Flesh target resistance

Flesh is a soft solid, that at high forces behaves more like a fluid, lacking considerable sheer strength. This is even true for lead when flying at rifles speeds impacting the games skin for the first 1 or 2 calibre penetration . The difference between a viscous fluid and a solid are the respective sheer strengths. A fluid may be moved with no later trace of the movement, whereas a solid withstands such try with sheer forces. In the case fast (> 400 m/s) flying lead into flesh, the interaction at these high forces is more like two penetrating fluids. Lead ist too soft to withstand impact pressures above 350 to 400 m/s undeformed.

Chamber target resistance
A shot through the chamber encounters mostly lung (when unfortunately the heart was missed). So depending on the animals size the incompressible chamber walls with a density rho ~ 1 g/cm³ are just one centimetre thick on the smaller game like Roe deer. Larger Game has slightly thicker rib wall. But from one side to the other most animal thickness is just filled with compressible light lungs. Opposed to flesh density of around 0,3 g/cm³ is only a third. Therefore the jam pressure at equal speeds is the same factor lower, a third. The destruction effect though, is even smaller, as lungs, opposed to soft solid flesh, are compressible. So the bullets front jam pressure, that applies large forces to the surrounding soft matter, reaches only short, as the lung is compressed. The lungs are designed to be compressed anyway.

Bone
Bones differ from flesh in two ways. First they are denser, exhibit more mass per volume than flesh an hence higher jam pressure. Second they are true solids, resisting deformation with sheer forces. Upon bullet impact bones brake into splinters, that, when ample accelerated, may rupture flesh next to it. Do not confuse this secondary effect with the lead bullet fragment destruction potential, as lead is by a factor of 10 denser than bone. So upon bullet impact accelerated bone splinters will, compared to lead splinters, only reach much shorter distances. The actual penetration depths are further below explained and calculated

Stabilization
Since the Gun was invented, Lead- or Stone balls are since many centuries obsolete projectiles. Long windslippery Bullets from Rifles replaced them. The main Reason to use long bullets is to increase the sectional density, that is mass per cross section, to leave less resistance to the air, while still moving enough mass per cross section into the target. If stable, longer bullets penetrate deeper.

Spin Stabilization
The jamming Air brakes long bullets at the Bow. The Mass pushes from the Mass center behind the Bow. To maintain its Axis in Flight direction, a long Bullets needs some Stabilization. Arrows use aerodynamic Loads at the End, after the Mass center, to achive corrective Momentum, that is to fly stable. Bullets from rifles use Spin stabilization. The Rifles in the Barrel follow a Helix. The Helix forces the Bullet to spin around its Length axis, as it accelerates along the Bore. Traveling in Air, the Spin stabilizes the Bullet. The tolerable Forces on the Jacket, limits allowable rotational Acceleration. If long Lead Bullets are shot from a rifle above about 400 m/s the Rifles cut the Lead off to the Core, hindering useful Rotation. Hard Jackets (Copper, Guild Metal, Tombak, soft Iron) are wound around the Lead core, to take higher rotational Accelaration, to allow to spin faster.

Flesh is about 800 times denser than air. Brake Forces correspond to the Density. As Air is compressible, but Flesh not, the Stagnation pressure on the Bullet bow, and so the Brake force there are in Meat a thousandfold higher than in Air. Spin stabilization for Air fails to stabilize such Bullet in the much denser Target. Depending on the Bullet shape, some Bullet will tumble upon Impact, reversing its Orientation such that the bullets heavier Part will move to the front and then Stabilize as such again. That corrective Momentum is the same as in Arrow stabilization, but this time turns the Bullet around. When a Bullet turn in the Target, it leaves direct Path, flips sideways off. When the bullet has a full metal jacket or is otherwise solidly constructed and slow enough, this orientation reversing behaviour is the case. Military rifle bullets yaw in tissue because their rotation maintains their point-forward travel in air, but insufficient to maintain that position in tissue - sooner or later they yaw to reach their stable centre-of-mass-forward attitude. Expect 7,62 mm military full metal jacket bullets at around 800 m/s to have moved to rectangular orientation towards flight direction after 25 to 30 cm in flesh.

Some bullets (the german 7,62 x 51 mm Nato round for instance) brake up, when they fly rectangular, as their exposed surface quadruples and jacket brakes in the middle at the weakest part, the crimp. The front, some 50% mass keeps together, while their open rear fragments to splinters. The faster little 5,7*45 mm Nato military bullets brake just from their speed from their fastest 970 m/sek down do maybe 820 m/s up, fragmenting more at high speeds and less at low speeds. Such fragmented military bullets, and their fragmented splinters move unstabilized. Well above 1.000 m/s all FMJ bullet fragment. The stuffs inherent pull strengths times crossection are too small to withstand the rifle bullets applied jam pressure forces:

Shoulder Stabilization
To shoot military bullets or hollow point bullets with a very tiny hole for match purpose such as Lapua Scenar or Sierra MatchKing on game is, for reasons that below will become clear, no prime choice to hunt big game. Better do not. Contrary to ordinary belief, when any semijacketed, being it round nose, sharp angled spitz or plain flat nose, lead bullet hits its target, more or less the same happens.

A 1 centimetre thick guild metal jacket could hold up to 4.800 Kp/cm². A 0,6 mm jacket will, according to its thinner wall, rupture earlier, that is at 0,6 mm / 10 mm = 0,06 times the pressure, that is at 288 bar. Or from 250 m/s speed on for a very small bullet. In practical hunt bullets around 5 to 10 mm calibre, due to diminishing pressure to the outside, only higher speeds will definitely rupture the jacket. As these coarse estimating calculations are filled with simplifying assumptions, true field test are needed to verify single data points.

Any how: Upon impact flattens the bow. See right picture as an examplary for all semi jacketed bullets. Stuff from a too sharp bow, that is not sideways supported, flows sideways off the bullet, kicking splinters into the flesh, until an equilibrium between forces is reached. The final bullet shape is always the same: flat round like a lens. The physical explanation are elliptical isobars in a flow onto a flat surface. Picture above the SakoSuper Hammerhead, already known in the 1920ies, then published by Prandtl, the father of all fluid mechanics.

Now we may discuss the bullets stabilization at its further flight in the target. The highest jam pressure exists at the bow middle. If the bullet tumbles a little bit, one shoulder moves forward, the opposite shoulder retards backwards. So the stagnation point moves on the bow a little sideways to the forward shoulder, generating the highest pressure there, now off the middle. The resulting momentum forces the tumbled bullet back into equilibrium. For the tumble beginning bullet the whole moving jam pressure point process  is called “shoulder stabilization”!

Every hunter observes, when he recaptures a fired semijacketed bullet, from game a typical bow shape, usually called Mushroom. Now one asks, how round the bow may be, to still guarantee shoulder stabilization. Consider a Sphere around a cylindrcal Bullet. The Mass center and the Sphere middle lies in the same Point. As the Shaft and Back are in a Vapour bubble, the Flesh touches Shaft and Back no more, the only Forces acting, are the Jam pressure and the braked Mass, against that Pressure over an Area. That is a Force as well. When in this specific Case the Bullet turns a bit around its Mass centere, the spherical Surface ramains.

So any Surface rounder, means further protruded than this, will destabilize the Bullet. Other flatter Surfaces will stabilize on their Shoulder. The best Bow for Shoulder stabilisation ist therefore flat, or the Bullet a Cylindre. Simple as that.

Roundnosed Bullets protrude their Middle much further, than the maximum allowable spherical Measure. Look at the right Picture. The Sphere, indicating the Radius above that no more Shoulder stabilization occurs, ist thin. The traditional Roundnose, a halph Sphere tangentially attached to a Cylindre, is shown a little thicker. The two Lines in an Angle, 35° to the Flight axis, show the Border between pushed Meat and Water vapour: The temporary Cavity lies inside. The Meat not even touches the full Roundnose.

The Roundnose will not shshoulder stabilze, as it has no Shoulder. Simple as that. Since the Spin stabilization is insuffcient in Meat, any Roundnose bullet will eventuallay tumble after some Penetration, then flip of its direct Path into Oblivion somewhere else. You simply cannot rely on a Roundnose, to keep it treck in Meat.

Caveat! If the Roundnose bullet is soft enough to flatten in the Target, then a different Bow may be hammered out; just likein any other semijacketed Bullet. So keep the the Marks to the non stable Roundnose strictly to the non deforming Solids or Monolithics. If you shoot onto thick Bone, then the Monolithic must be of Brass. Anything less stiff will flatten in Bone at usual Speeds

Remember: Any bullets bow needs to be flat as a saucer or a lens to yield shoulder stabilization: else it tumbles.

Target effect

Full metal jacketed military bullets or tiny hollow point bullets (close to being full metal jacketed bullets) won't fragment at moderate speeds, lead won't flow off, won't develop the lens shape, to allow shoulder stabilization, therefore after passing some 10 to 15 narrow funnel, tumble, and later restabilize until their heaviest part flies in front, reversing original flight orientation. To reach the new stable force equilibrium may well take a half meter.
 

Semi jacketed lead bullet

So what happens to the target, when a soft pointed rifle bullet hits the skin? At speeds above, say 800 m/s the jam pressure upon impact presses lead sideways off the bullet into the flesh. The big “mother bullet” breeds tiny leaden “daughter bullets”. Such lead splinters usually size between 1 and 3 mm spare and are maybe a mm thick, weighing between 10 and 90 milligram (0,01 – 0,09g). Being still either fast those little bastards are by no means harmless. The still may punch holes into flesh or much more into the light soft compressible lung. Any bone will stop those quick within mm depth. Breeding the daughters the mother looses weight. This lost weight mostly depends on two factors. First important for splinter generated weight loss is how fast the primary bullet hit. Second important for fragmentation is how thick the jacket is built (and made of iron, or Tombak, or else). Bonding the jacket to the core (through soldering or other means) makes in the fragmentation no difference, as the soft lead cannot hold the sturdy jacket. The assistance works only the other way around, as only the strong can hold the weak , not vice versa. The daughter breeding happens usually within the first 2 to 4 cm after the skin. Then the mother is flattened, weight reduced, shoulder stabilized  and flies with lower through the target until it stops or has on the other side still ample speed to exit through the skin again. The required exit speed varies with game but is around 50 m/s. Below the required exit speed the bullet hits the skin like a boy a trampoline and just bounces back, remains inside the body. Splinter weight may be 20% mother weight for fast soft bullets and less for slower sturdier ones.