Exterior ballistics is just a sciencey sounding word that is used to describe anything flying through the air. Could be bullets; could be snow balls.
This is a very math intensive area, but I’m going to do my very best to keep the math out of it as much as possible.
Once a bullet leaves the barrel of a gun, or a ball leaves the hand that threw it, it is at the mercy of physics. Where it goes is determined from the moment it breaks contact, unless the wind messes with it.
Look at a baseball. When an outfielder throws the ball to second base, he doesn’t throw it at the second baseman. It would never get there. Instead, he throws it up, in the direction of the second baseman. Air resistance slows the speed of the ball and gravity pulls it back down. The same thing happens to bullets.
If a bullet is fired exactly level with the ground, it will start to drop the instant it leaves the barrel. It will actually hit the ground at the same time as a bullet that is just dropped from the same height at the same time.
We need to define a new term here. LINE OF SIGHT is a straight line drawn from the rear sight of the gun to the front sight of the gun, and extended out to the target and beyond. If I aim a pistol at a mountain top 10 miles away, I’m never going to hit it, but I have a line of sight to the peak. In that same way, if I aim a rifle at a target 100 yards away, I have a line of sight from the sights of the rifle to the target.
The bullet will actually leave the barrel an inch or more below this line of sight, fly up past it, level off and drop back down, crossing it a second time. If the gun is sighted in exactly right, the bullet will cross the line of sight at the exact range of the target.
Because this is all physics, the amount a bullet will drop can be calculated without ever having to fire it. There are a lot of variables in this, but the main ones are the speed of the bullet when it leaves the gun, and the ballistic coefficient, which is a fancy way of saying “the shape of the bullet”.
The Ballistic Coefficient, or BC, is a measure of how well the bullet moves through the air. A higher value means the bullet slows down less per unit of time. This is one of the reasons that a rifle bullet is longer for its diameter than a pistol bullet. Generally speaking, a long narrow bullet will be better for long range shooting than a short, fat bullet. Being longer also means they are heavier, but that is actually not as important for how they fly.
So, how does all of this affect you as a writer?
Let’s start with the laser sight cliché. A laser beam travels in a straight line. It’s a physical representation of the line of sight. The bullet will be close to that line for a while, but once it drops below it, it will continue to move away until it hits the ground. The only time a laser sight will really indicate where the bullet will hit is if the gun is sighted in for that range.
It’s not all Hollywood though. Because the bullet travels in an arc, it’s possible for the line of sight and the bullet path to be within a couple of inches of each other for 50 to 100 yards. Beyond 100 yards, even rifles will have a noticeable drop from the aim point.
Somewhere in the 90’s a memo went out that said that holding a handgun sideways was cool. I’m not sure where I was at the time. Possibly at a range actually hitting what I was shooting at. The result was a deluge of TV and movie characters that tipped their handguns 90 degrees to one side. A Right handed shooter would hold the gun with the sights to the left side rather than on top, and the bottom of the gun would face the right side. Here’s why this is so bothersome to those of us that know how guns work.
Remember how the gun fired the bullet up slightly? It does that because the barrel is pointed up slightly. When the gun is tipped to one side, that slight angle is no longer up, but points to the left (for a right handed shooter). This change means that the bullet will never climb after leaving the barrel, and will instead hit low left, often by a surprising amount.
Now, people who actually practice shooting this way can eventually compensate for it, but the sights will be useless.
Guuuns in Spaaaaaaaace. (Sorry, Muppets)
With no air to slow the bullet, and no gravity to pull it back down, a bullet fired in space will travel in a straight line.
Eventually it will get too close to something and change paths, but the initial inertia of the bullet will virtually guarantee that the bullet will pass higher and higher over the target as the distance increases. For a planet or moon, there will be gravity, and possibly air resistance that can greatly affect the path the bullet takes. If you plan to have a gun in a non-Earthlike environment, be aware that certain changes will be needed to fire accurately.
For a police procedural type of story, the shape of the bullet can be very important. A 7.62 mm hole could be from a sniper rifle or from a small semi-automatic pistol. If the bullet is long, it eliminates the pistol as a possibility. Interestingly, a short bullet does not eliminate the possibility of the weapon being a rifle. In theory, the .71 grain bullet from a .32 ACP could be loaded into the case of a 30.06 hunting rifle. Accuracy would be an issue since the rifling twist would be all wrong, but for a close range killing, it wouldn’t be insurmountable.