I recall a MythBusters episode in which they were trying to determine if falling bullets would be dangerous. Rather than trying to catch one and measure its velocity coming down, they determined what (they thought) was their terminal velocity by putting some handgun bullets in an air stream and seeing how fast it had to blow to keep them suspended.

I contrasted that method with something I’d read many years ago about experiments after World War I to determine how high rifle bullets would go if fired straight up. One of the things I remembered about that effort was the discovery that bullets (those, anyway) didn’t tumble and if fired mostly straight up they came down base first. That was determined by the method of timing and finding them when they came down. The tests were conducted over smooth beach sand that made locating the impact holes and recovering the bullets relatively easy.

And the reason they remained stable in the same orientation up and down? Because although their movement up through the air slowed and stopped due to drag, spin speed decreases much more slowly and was evidently fast enough to keep the bullet point up even while falling back to earth. If a rifle bullet fell in a stable orientation base down, I would expect its terminal velocity to be much higher than if it were tumbling.

Yes a casing, or a projectile would have enough energy to kill a human, if fired from altitude. I don’t have the math anymore but I know it will. When we fired 76mm (3inch) antiair projectiles, wether they impacted a plane, blew into fragments like they were designed to do or completely failed and became big steel bullets, they most definitely would kill anything they impacted. A 50 caliber bullet is around 650- 700 grains and they would be heavy enough to kill you and an empty 50 case dropped from 5k feet would hit you just like the projectile would, dead.

quote:

Originally posted by sigfreund:
I recall a MythBusters episode in which they were trying to determine if falling bullets would be dangerous. Rather than trying to catch one and measure its velocity coming down, they determined what (they thought) was their terminal velocity by putting some handgun bullets in an air stream and seeing how fast it had to blow to keep them suspended.

Sounds good, but if a column of air is keeping the bullet suspended in place, it's not the same as terminal velocity. A bullet suspended in place on a column of air is still only subject to one gravitational value, with no acceleration to counter the air stream. The air stream only counters the mass of the bullet. The kinetic energy of the bullet is effectively zero as it's stationary.

In freefall, the bullet is accelerating downward at the rate of 32 feet per second, per second, standard gravitational constant for acceleration (or 9.8 meters/second/second, if preferred). Mythbusters is suggesting that essentially if the airflow velocity were increased more than their stationary bullet experiment, the bullet would move back up, and thus their air velocity equals terminal velocity. That only applies to the stationary bullet, however, and not to the force that would be required to counter the bullet in freefall.

The necessary drag rise could be calculated if assumptions were made about the fall angle of the the bullet and thus it's drag coefficient, which rises at the rate of the square of the airflow velocity; that value could be leveraged against gravitational acceleration to come up with a terminal value, but we know the bullet wouldn't be stable in that position and the value would be theoretical.

I know from spending time in freefall that my body position has a great deal to do with terminal velocity. If I am stable, face-to-earth, for example, I'll reach about 130 mph. If I assume a head-down position, however, I can accelerate to approximately 210 mph, and feel it. If I return to the basic face down frog position, I'll feel myself slow rapidly back to that terminal velocity of 130 or so. The type of clothing worn (tight jump suit, street clothes, loose draggy jump suit, wing suit, etc) will affect freefall in a big way, as will wearing a weight vest, or simply arching my back or making myself larger or smaller (more or less drag). Clearly there's a lot of variability involved.

quote:

Originally posted by sigfreund:

I contrasted that method with something I’d read many years ago about experiments after World War I to determine how high rifle bullets would go if fired straight up. One of the things I remembered about that effort was the discovery that bullets (those, anyway) didn’t tumble and if fired mostly straight up they came down base first.

Our rule of thumb was 4,000' above ground as safe altitude from small arms fire, or that 4,000 represented an altitude from which small arms fire wasn't considered a threat.

Safe is a relative word.

My brother caught unholy hell in VN after dumping the brass out of his helicopter after a mission. It seems a major flying a plane below him had his plane damaged. This was minigun brass (7.62). I would think 50 cal brass could possibly kill someone. It is pretty heavy.