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January 7, 2008, 01:51 AM   #20
JohnKSa
Staff

Join Date: February 12, 2001
Location: DFW Area
Posts: 21,964
Quote:
 There is no way a bullet fired straight up in the air is going to be any more lethal than a hailstone of the same size.
Incorrect.

An object falling through air is subject to two forces, drag and gravity. Terminal velocity occurs when the two forces are balanced. A hailstone the same size as a bullet would have about the same drag, but the force of gravity is greater on the bullet since it is more dense (lead is denser than ice) and therefore weighs more (weight is the force on an object due to gravity). Therefore a bullet would fall faster than a hailstone of the same size.

Furthermore, a spin-stabilized bullet will fall either base or nose first rather than tumble. That reduces its drag considerably which means it will fall at a higher terminal velocity, perhaps double the velocity it would fall if tumbling. The tests I've seen suggest that falling pistol bullets will tend to tumble while falling rifle bullets tend to remain stabilized and fall back to earth base first.

Comparing bullets to hailstones is not an accurate way to assess the lethality of falling bullets.

It's kind of interesting that you make this comparison. Sometime ago I calculated the energy and velocity of various size hailstones. A local company was replacing their roof coating and wanted to know how to test it for hail resistance. One of the facilities personnel is a friend of mine and asked me to verify if the test that had been suggested to them was valid. (It involved dropping a steel ball bearing of specified weight from a specified height.) I did some research and calculations and determined that the test was valid based on the fact that the ball bearing imparted the same impact energy to the roof as a hailstone of the size that the roof material was rated for would. The embarassing part is that I enjoyed it. Oh well.

You are correct that a bullet that takes an arc-shaped trajectory (not fired exactly straight up) will come down at a much higher velocity than a falling bullet. In that case the velocity of the bullet is the vector sum of the terminal velocity and any retained horizontal velocity not lost to air resistance. Furthermore, in this case, the bullet will tend to remain spin-stabilized and traveling nose first which means the terminal velocity will be maximized due to the fact that drag is minimized. Definitely a recipe for disaster.
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