Topic of the Month: March
2001
Bullets in the Sky
We frequently get questions about firing bullets vertically
into the air. The most frequent question is, "Will bullets fired into the air return
to the earth at the same speed they left the gun?" Other questions asked are;
"How far does the bullet travel when fired vertically and how long does it take to
come down, or does the falling bullet have enough energy to be lethal should it strike
someone on the ground?"
Some have tried vertical shooting, but very few have had any
luck hearing the bullet come back and strike the ground. When a bullet is fired vertically
it immediately begins to slow down because of the effects of gravity and air drag on the
bullet. The bullet deceleration continues until at some point the bullet momentarily stops
and then it begins to fall back toward earth. A well-balanced bullet will fall base first.
Depending on bullet design, some bullets may tumble on their way down and others may turn
over and come down point first.
The bullet speed will increase until it reaches its terminal
velocity. The bullet reaches terminal velocity when the air drag equals the pull of
gravity or stating it another way, the bullet weight and drag are balanced. Once this
velocity is achieved the bullet will fall no faster.
In 1920 the U.S. Army Ordnance conducted a series of
experiments to try and determine the velocity of falling bullets. The tests were performed
from a platform in the middle of a lake near Miami, Florida. The platform was ten feet
square and a thin sheet of armor plate was placed over the men firing the gun. The gun was
held in a fixture that would allow the gun to be adjusted to bring the shots close to the
platform. It was surmised that the sound of the falling bullets could be heard when they
hit the water or the platform. They fired .30 caliber, 150 gr., Spitzer point bullets, at
a velocity of 2,700 f.p.s. Using the bullet ballistic coefficient and elapsed time from
firing until the bullet struck the water, they calculated that the bullet traveled 9,000
feet in 18 seconds and fell to earth in 31 seconds for a total time of 49 seconds.
As a comparison, the .30 caliber bullet fired in a vacuum at
2,700 f.p.s. would rise nearly 21.5 miles and require 84 seconds to make the ascent and
another 84 seconds to make its descent. It would return with the same velocity that it
left the gun. This gives you some idea of what air resistance or drag does to a bullet in
flight.
Wind can have a dramatic effect on where a vertically fired
bullet lands. A 5 mile per hour wind will displace the 150 gr. bullet about 365 ft based
on the time it takes the bullet to make the round trip to earth. In addition the wind at
ground level may be blowing in an entirely different direction than it is at 9,000 feet.
It is no wonder that it is so difficult to determine where a falling bullet will land.
Out of the more than 500 shots fired from the test platform
only 4 falling bullets struck the platform and one fell in the boat near the platform. One
of the bullets striking the platform left a 1/16 inch deep mark in the soft pine board.
The bullet struck base first.
Based on the results of these tests it was concluded that the
bullet return velocity was about 300 f.p.s. For the 150 gr. bullet this corresponds to an
energy of 30 foot pounds. Earlier the Army had determined that, on the average, it
required 60 foot pounds of energy to produce a disabling wound. Based on this information,
a falling 150 gr. service bullet would not be lethal, although it could produce a serious
wound.
Many other experiments have been made to find the amount of
air drag on a .30 caliber bullet at various velocities and it was found that the drag at
320 f.p.s. balances the weight of the .021 lb. (150 gr.) bullet and terminal velocity is
achieved. For larger calibers the bullet terminal velocity is higher since the bullet
weight is greater in relation to the diameter. Major Julian Hatcher in his book Hatcher’s
Notebook estimates that a 12 inch shell weighing 1000 pounds and fired straight up
would return with a speed of 1,300 to 1,400 feet per second and over 28 million foot
pounds of striking energy.
Watch our web site for the next topic of interest "How
Far Will My Gun Shoot." Until then, shoot safely and know where you bullets are
going.
Sincerely,
The Ballistician
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