Scenario Consultation - Guns, Cars and Night

A writer posed the following scenario to me:

• Protagonists are driving along a rural Colorado road, outside Denver, late at night
• There are houses, on wooded lots, no closer than ¼ mile to each other
• Terrain is gently rolling hills
• Very Bad Guy (VBG) shooter is outside, covering the road with a rifle
• Weather is clear and dry
• There is a ¾ moon with very occasional cloud obscuration
• Shooter is a former military sniper
• Money is not an object as far as the shooter’s equipment goes
• Shooter will leave the area after shooting by car
• Shooter is to shoot the car through the rear window, with the round to exit the windshield, leaving a starred hole
• Distance is between 100 and 300 meters
• Shooter is using a night vision device

 Initial Questions from Writer

  

a.       What kind of rifle and ammunition would the shooter use? 

b.      Would the impact of the bullet spin the car or cause loss of control? 

c.       Would the round disappear and be impossible to find?

Analysis Part 1 – Location, Location, Location

There are few more things at issue here than choice of weapon.  Specifically:

  

a.       Concealment:  Our shooter will want to lie in wait in a place where he’s not readily observed. 

b.      Position relative to the car:   In order for the round to travel through the car as desired, the shooter needs to be directly behind the car, and at a comparable elevation.  If he’s off to the side, and the round will exit through a side window (or a passenger).  Too high and the bullet will exit through the bottom of the car.  Too low, and the bullet could go out the roof.  

c.       Speed:  Ideally, the car will be standing still so that an accurate distance calculation can be made, but due to the scenario constraints, that can’t happen, so our shooter needs to position himself in a place where the car will be moving as slowly as possible.

Solution Part 1

The shooter needs to position himself in a treeline, behind a bend in the road.  For our purposes, let’s make this a two lane (one in either direction) rural road.  The treeline (and the darkness) will provide him a concealed shooting position, while the 90 degree bend allows him to be directly behind the target vehicle without being in the roadway itself.  Importantly, the 90 degree bend also forces the car to slow down to between 10 and 20 miles per hour, and then accelerate back to highway speed.  This period of slow movement provides the shooter a window in which he can engage the target when it at a known distance.

Why is a known distance so important?  Because of a little thing called GRAVITY.  Bullets do not follow a straight path, like a laser.  Instead, they fly in a ballistic arc, dropping in elevation as they get further from the gun’s muzzle.  Shooters account for this by adjusting sights such that in order to look directly at the target, the firearm’s muzzle has to be raised to some degree above the horizontal, launching the bullet upwards so that it drops into the target.  As an example, at a distance of 200 yards from the muzzle, a bullet fired from a rifle chambered for the .308 Winchester cartridge will drop almost eleven inches.  By knowing the distance at which the shot is to be taken, the shooter can pre-set the range dials on his sights, compensating for drop over distance. 

A schematic of an ideal setup for this scenario is shown below.

Analysis Part 2 – The Right Tools

There are four components to tool selection:

Target

Ammunition

Optics

Rifle

Interdependencies between all of these come into play for our shooter.

a.       Target

The target is a car.  More specifically, it is a car WINDOW.  Glass (tempered or otherwise) is an interesting substance.  It’s extremely hard, and extremely brittle.  Sure, it will shatter when struck, but it will also have very significant effects on projectiles as they pass through.  A bullet in a typical hunting caliber (let’s say .308 Winchester, .30-06 or .30-30) will pass through, but will also be deflected unpredictably.  There is also a significant chance of large scale deformation.  It might continue straight, it might tumble, it might move up, down, left or right.  As a result, the nature of the target has an impact (no pun intended) on our choice of ammunition.

b.      Ammunition

Given the nature of the target and the potential effect it will have on projectiles, we can deduce a requirement for a projectile that will have minimal, if any, deflection when moving through read window glass.  This narrows the selection scope to big, heavy, high speed projectiles.

c.       Optics

It’s dark, but we have a relatively large amount of ambient light.  What our shooter needs is some sort of optic that will intensify the ambient light and allow him to see his target clearly.  Fortunately, there are many optics built around image intensifier tube technology.  Image intensifier tubes convert low levels of light from various wavelengths into visible quantities of light at a single wavelength.  This technology is available in many precision firearm optics.

d.      Rifle

The easy part, right? It’s got to be able to a) use a large cartridge that features a heavy projectile and b) mount an image intensification optic.

Solution Part 2

It’s a good thing that our shooter is a) well heeled and b) a former military sniper.   The former means that we can kit him out with the objective solution system without worry about cost, while the latter means that he can plausibly have experience with the objective system.  Without further ado, let’s introduce “the right tool for the job.”

Barrett M82A1 (M107) with AN/PVS-10 SNS

The current US military long range sniper rifle is the Barrett M82A1, type classified by the Army as the M107.  It is chambered for the .50 Browning Machine Gun (BMG) cartridge, and a number of specialized sniper rounds have been developed for it.  Among these is the M1022 long range sniper cartridge, with an effective range of 1,800 meters.  It uses a 670 grain (about 1.5 ounces) projectile with a muzzle velocity of about 2,800 feet per second.  This means that at the 250 meter distance to the target in our scenario, the bullet will strike the window moving at around 2,450 feet per second with an energy of some 8,950 foot pounds.  That’s a LOT of foot pounds.  To put this in perspective, Dirty Harry’s .44 Magnum had an energy at the muzzle of about 1,200 foot pounds.  To get an idea of just how truly huge the .50 BMG is, take a look at the photo on the left.  The cartridge on the left is the US military 5.56mm, used by the M16 series of rifles.  The center cartridge is the .338 Lapua, considered a large, high performance round.  The .50 BMG is on the right.  As my Canadian friends might say, "Big, eh?"

For our scenario this means that the projectile will punch through the rear window as though it isn’t there and zip right through the windshield, creating a half-inch hole with lots of nice star shaped craze cracking.

IR ChemLites

AN/PVS-10 Sniper Night Sight

I know what you’re thinking.  But Adam, it’s DARK!  How is he going to see the car?   I saved the best for last.  The M107 is frequently fitted with the AN/PVS-10 Sniper Night Sight, which  couples image intensification technology with a 12.2 power telescopic sight.  Using this sight, the shooter will be able to see the target clearly, and, at 250 meters, be able to select the part of the window he wants to hit.  The icing on the cake is the distance determination.  By putting an infrared ChemLite out on the road at precisely 250 meters from his shooting position, he’ll be able to know when the target is at the proper distance.  Infrared ChemLites emit a glow that is invisible to the naked eye, but clearly seen with an image intensification device.

There are one or two other issues, such as the five to six foot width of the car, and the fact that about half of that is taken up by the occupants - but those are details that can be plausibly explained in the story.

Summary

Now that we've got our shooter set up and equipped to handle the physics of the scenario, we can identify the way things happen.  The sequence of events works like this:

1. Earlier in the day, our shooter selects a concealed position in the woodline behind the bend in the road.
2. The shooter identifies a spot on the road 250 meters from his shooting position, and sets a 12-hour infrared ChemLite on or next to the road, where he can see it from the shooting position.
3. Shooter takes up position.
4. Sometime after dark, shooter hears car approaching from his left.
5. Shooter identifies car as target, and waits for car to slow down and turn at the bend.
6. Shooter acquires target.
7. When the target passes the infrared ChemLite, shooter fires.
8. M1022 bullet strikes rear, tempered glass window, which shatters into many small but harmless pieces.
9. Bullet continues, exiting  through a single hole in the laminated, safety glass windshield, and disappears in the distance.
10. Stunned/alarmed by the sudden shattering of glass, driver loses control and car goes into shallow ditch on left side of road.
11. Shooter, observing target, confirms that there will be no pursuit.
12. Shooter retrieves fired case, picks up rifle (about 35 pound), and returns to his car (see diagram).
13. Shooter separates upper and lower receivers, halving the length of the rifle, puts disassembled rifle in trunk, and drives off.