Stress_Test remarked,
I read a book called "American Rifle" that had a chapter about the M16. Apparently, some of the early M16s were mistakenly produced with a ridiculously slow twist rate, like 1 in 20 or something. This meant that the bullets were basically wobblin' goblins in flight, and when they hit a bad guy, the bullet would immediately tumble out of control, creating major wound trauma.
The spin required to stabiize a bullet varies with the density of the medium through which it is passing. More dense materials, such as flesh, require more spin to stay point-on. But the twists we find in most firearms are designed to be for that ammunition in air. So they tend to destabilize when entering other materials.
Like flesh.
How much of this "tendency" is the subject of much experimentation in Ordnance Departments around the world.
For more dense target materials, "the spin as worked out for air must be multiplied by the square root of a number found by dividing the density of the material in question by the density of air." (
Hatcher's Notebook, p 556-557 and following.)
This, bearing in mind that there are no sharp cutoffs in twist calculations, such as the "Greenhill Formula," for
estimating twist requirement for a particular bullet. In general, it's better to have a faster twist so that longer (hence heavier) bullets can be used if necessary.
Another destabilization factor is the "arrow effect." I understand, but cannot cite offhand, that certain variations of combloc bullets had the points virtually hollow, with the main weight of the core toward the rear of the bullet to encourage tumbling in denser materials.
Like flesh.
Point shape is also a consideration.
Yet another major factor is the density of the bullet. A greater twist is required for less dense bullets. An aluminum bullet of the same size and shape of a gilding-metal clad lead bullet would have to have twice the spin required for stabilization in air. (Op cit, p 556)
The new developments in 5.56 NATO bullets are a result of experimentation to determine the best compromises between accuracy at range and terminal ballistics (including armor penetration
and destruction of tissue) out of the short-barreled M4 carbine.
Terry, 230RN