fourfive8
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This is an interesting subject that leads to long discussions. 
The sectional density parameter as it relates to potential penetration is valid only for as long as the bullet remains undeformed along its path at the ballistic terminal end of travel. Sectional Density = Mass /Diameter (squared). The other variables that are difficult to pin down, analyze and predict are spin stabilization and mass forward of aerodynamic pressure stabilization within the dense media at the terminal end. Very generally I think momentum is the parameter that best describes the penetration potential for a bullet compared to other bullets with similar SDs. But again it's not a perfectly simple comparison because so many other variables may be at play between any two bullets.
One of the buggers in trying to predict or even fully understand bullet behavior, scientifically/mathematically, is the real but mostly overlooked conflict between the concepts of kinetic energy and momentum. As it turns out energy, as defined, is usually the enemy of penetration while momentum is the friend. By definition, energy is two parted- the harder a bullet pushes on any substance, air or more dense material- doesn't matter, the harder the substance pushes back (and the results of that push back is seen in the deformation of the bullet). And that kinetic energy concept is why even a little 22 rf will leave a relatively huge crater in wet clay. While at first glance energy and momentum seem to be very closely related (or even the same) they are in reality two very different things that have to be looked at differently in the context of ballistics.
What all this means in practical terms- I dunno? other than it is interesting to think about. And sometimes you can look at bullet behavior and, at least in some small way, understand why the bullet did what it did. Or, given some parameters of design, possibly predict a behavior.
Then what's most interesting to me is actually testing a bullet in media and comparing that to what the prediction is. There are always surprises along the way. One example: I was testing a large caliber, near pure lead cast bullet at very modest impact velocity in wetpack media. The penetration was far greater than I had predicted and equalled the very best of high velocity, jacketed controlled expanding bullets.
The sectional density parameter as it relates to potential penetration is valid only for as long as the bullet remains undeformed along its path at the ballistic terminal end of travel. Sectional Density = Mass /Diameter (squared). The other variables that are difficult to pin down, analyze and predict are spin stabilization and mass forward of aerodynamic pressure stabilization within the dense media at the terminal end. Very generally I think momentum is the parameter that best describes the penetration potential for a bullet compared to other bullets with similar SDs. But again it's not a perfectly simple comparison because so many other variables may be at play between any two bullets.One of the buggers in trying to predict or even fully understand bullet behavior, scientifically/mathematically, is the real but mostly overlooked conflict between the concepts of kinetic energy and momentum. As it turns out energy, as defined, is usually the enemy of penetration while momentum is the friend. By definition, energy is two parted- the harder a bullet pushes on any substance, air or more dense material- doesn't matter, the harder the substance pushes back (and the results of that push back is seen in the deformation of the bullet). And that kinetic energy concept is why even a little 22 rf will leave a relatively huge crater in wet clay. While at first glance energy and momentum seem to be very closely related (or even the same) they are in reality two very different things that have to be looked at differently in the context of ballistics.
What all this means in practical terms- I dunno? other than it is interesting to think about. And sometimes you can look at bullet behavior and, at least in some small way, understand why the bullet did what it did. Or, given some parameters of design, possibly predict a behavior.
Then what's most interesting to me is actually testing a bullet in media and comparing that to what the prediction is. There are always surprises along the way. One example: I was testing a large caliber, near pure lead cast bullet at very modest impact velocity in wetpack media. The penetration was far greater than I had predicted and equalled the very best of high velocity, jacketed controlled expanding bullets.
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