Saturday, January 21, 2012

Scenario Consultation - Shotguns and Cars

A writer posed the following scenario problem to me:

“So what kind of damage can someone do with a shotgun accidentally fired from a front porch to a car parked across the street, a distance of maybe 50 yards or less?

Cozy mystery, so no occupants and little bloodshed. I would like the windshield destroyed, the car alarm to go off, and if possible some damage to the body of the car, but not a huge amount damage. I would like the owner of the car to have the windshield fixed but decide it's just not worth fixing the rest of the car. I want him to notice the damage every time he looks at his car.
The scene has a very frightened man who has had too much to drink fall backwards and as a result the gun goes off. “

Bottom Line Up Front: The shotgun will shatter the side windows and punch a couple of holes in the door. No pellets exit the car, so there is no danger of collateral damage.

Detailed Scene Analysis

1. Setting the Scene

Perhaps the most important thing is understanding the physical relationship of the actors in this scene to one another. We have two actors: The shooter and the car. They are separated by some linear distance – let’s say about 25 yards to account for the average four lane, two way residential street (parking lane – travel lane – travel lane – parking lane) as well as a porch near, but set back from the street. Given that, we have something like this:

Shotgun Scene Physical Schematic

Note a couple of things:

a. While the author originally said that she wanted the windshield destroyed, I modified the scenario to destroy the side windows instead. This is to ensure that the damage to the car was, as she wanted, cosmetic and not mechanical, as it might be if projectiles struck the engine area, which they might if the car was shot from a direction from which the windshield might get hit.

b. In the lower (side) view, note that the projectile path angles upward as it moves toward the car. Remember, the shooter has either fallen or dropped the shotgun, so firing is effectively done from ground level.
2. Relevant Firearm Data (Shotgun)

a. Ammunition

Shotguns fire two basic types of ammunition, shells and slugs. A shell contains a number of projectiles, which can range anywhere from nine 0.31” pellets in a “00 Buckshot” shell, to some six hundred 0.08” pellets in a “Number 9 Birdshot” shell. A slug is a unitary “BFB,” or “big friendly bullet,” usually lead. For this scenario, our shooter’s shotgun will be loaded with 00 buckshot, which allows for up to nine separate impact points on the vehicle.

b. Mechanics

Shotguns, like most firearms, fire their ammunition by causing a steel punch (firing pin) to strike the base of the shell where the primer is. This firing pin is, in most cases actuated by a spring loaded hammer impacting the base of the firing pin as it rotates forward under spring pressure. The hammer’s rotation is held in check by a mechanical piece called the SEAR (hold on to that term!), which is the part that is acted upon by the trigger when it is pressed by the shooter. An age-worn or damaged sear can cause a sudden deceleration, as happens when a firearm is dropped, to allow the hammer to slip from the sear’s grasp and unintentionally/accidentally fire the shell in the chamber.

In the case of a manually operated shotgun, like a pump or lever action design, there will be only a single round in the chamber, limiting an accident to the discharge of a single shell.

3. Firearm Selection for the Scenario

Since this is an accidental shooting, we want to limit the shooter to one discharge. This militates toward the use of a manually operated gun, like a pump action. Additionally, we want the gun to be in a condition of wear and age where it is plausible for a sharp blow to release the sear. For these reasons, I recommend that our shooter carry an Ithaca Model 37 “Deerslayer” shotgun of early 1950’s vintage, possibly a police surplus gun. Ithacas are strong, reliable guns that, with care should outlast generations of shooters. They were very popular with police departments all over the country. However, police guns aren’t always maintained as well as they could be, allowing for the possibility of an old or worn (or both) sear.

4. Damage 

Shotgun pellets shed velocity and energy very rapidly when they hit a target such as a car. They’ll penetrate one door and either stop in the door, or somewhere in the far door. Typical door damage from 00 Buckshot looks like this:

Note that the large hole to the right is from a slug.

Shotgun pellets will, of course, shatter a car window.

5. Scenario Summary

Based on the information at hand, the sequence of events, and results goes something like this:

a. Drunk protagonist comes out of house with old Ithaca M37 12 gauge shotgun.

b. Protagonist chambers a shell.

c. Protagonist trips and falls and/or drops gun

d. Gun has worn sear, and as a result, the impact jars the hammer loose from the sear and the gun fires.

e. Pellets move across the street toward parked car in a generally upward angle.

f. Seven pellets impact window(s) and shatter them, coming to rest inside car. Two pellets punch holes in door, coming to rest inside the far side doors without exiting

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