The body of human being as well as animals contains in average 70 % of water.
The water content in muscle tissue and especially bone tissue is much lower than the average which means that internal, vital organs contain even higher amount of water
Every bullet travels in a cavity when the target is soft organ tissue containing high amount of liquid
The size of the cavity is defined by the shape and size of the projectile as well as the speed it travels trough the tissue
If the projectile is full metal jacketed (FMJ) "Spitzer" the time required for the tissue to move away from the path of the projectile is relatively long and only extremely high speed projectile of this type can cause big wound channel
This type of projectile is always very unstable when traveling in body tissue because the center of the gravity is far back compared to the geometrical center of the bullet and nothing is supporting it inside the cavity except gyro force.
In most of the cases this type of bullet starts to tumble which stops the bullet quickly. Tumbling bullet causes quite small wound channel and is subjected to high friction force creating mainly heat
Next to the sketch a real bullet which travelled only 30 cm in wet phone books and started to tumble already after 20 cm penetration. Note that the lead insert has been squeezed out during tumbling
If you shoot this type of bullet on wood where it does not form a cavity and it is supported by the wood it will penetrate deep
When the bullet expands to double diameter mushroom the shape is generally half round
The time for the tissue to move away from the bullet path is greatly reduced. When also the volume of the mass to be moved is increased the cavity formed by the projectile is greatly increased.
Because the cavity is bigger, also the cavitation after the bullet is increased causing tissue damage
The big frontal area and curved shape increases the friction force generating heat and slowing down the projectile fast
Reduced speed of the projectile increases the time for the tissue transfer and the formed cavity decreases in size as the speed is reduced
The result is big wound in the surface of the animal while the damage in the vital organs is smaller. The design and material of the bullet as well as the impact speed has great influence on the wound.
A rule of thump is that the faster the impact speed the bigger surface damage and less penetration.
Penetration is normally sufficient on all thin skinned game. However on dangerous game this type of bullet can be used only for a broad side shot and it is also recommended for smaller game because of limited effective penetration.
This type of bullet is very stable as long as the shape of the mushroom is uniform, because the centre of gravity is far in front of the projectile. If the mushroom is fragmented the bullet may tumble or change direction greatly inside the tissue and fragments can travel in unpredictable direction causing tissue damaged and even danger
If we make the projectile cylindrical
shape flat nose the time required for the tissue to be moved closes
zero. In theory this means that the speed of the tissue moved is closing
In real life this is not of course achieved and tissue is rather crushed causing bigger primary wound channel.
Actual testing has shown that caliber diameter flat nose creates surprisingly big wound channel though smaller than double diameter mushroom at same impact speed because the amount of tissue moved is clearly smaller
Second finding we have proved by testing both theoretically on wet phone books as well as in real hunting situations is that the size of the cavity caused by this kind of projectile is much more bound to the tissue water content. The high speed of tissue transfers the shock wave which creates preliminary cavity, much further in high water content vital body parts, than in lower water content muscle tissue. This reduces greatly the meat damage and increases killing power.
Because the frontal area is small also the friction force is lower which enables the projectile to maintain it's speed and capability to create 3 - 3,5 x longer wound channel than corresponding double diameter mushroomed projectile.
This long effective wound channel enables more demanding shooting angles than only broad side
If this projectile is shot on wood it will have relatively short penetration because it can not create cavity in the wood.
The killing power of this projectile is enhanced because it travels trough vital organs at much higher speed than traditional projectiles
This projectile is extremely stable because the center of gravity is in the geometrical center and the flat nose stabilizes turning forces resulting predictable straight line penetration