Understanding Basic Bullet Ballistics

Understanding Basic Bullet Ballistics

Bullet Ballistics

Even if you never do long range shooting, and just go plinking once a year, you should understand bullet ballistics.  Basic understanding of bullet ballistics will not only help understand how to sight in your guns.  It helps you understand the reason for different bullet weights and types.  This helps hunters pick the right ammo for the caliber they own.  Plus it helps reloaders pick the right bullet for their application.  Understanding basic bullet ballistics helps with anything you do with a gun.

Basic Definitions:

You will need to understand a few basic definitions before you can learn bullet ballistics.  Just like many people interchange a gun magazine with a clip.  If you don’t know some basic definitions from the shooting world.  You will have a hard time understanding the important concepts.

Ammunition / Cartridge

CartridgeThe terms ammunition or cartridge are rarely used amongst people.  People always just call them just bullets.  A bullet is just component of a cartridge.  A cartridge is the whole thing you load into a gun to fire it.  When you go to a store to buy a box of ammunition.  It will have a set number of cartridges in it.  The cartridge consists of a bullet, a case, powder and a primer.  They are all put together to form a cartridge you load into a gun to fire the bullet out the end.


The bullet is fired out the end of the gun.  This does not include the case.  It is a projectile.  However, no one calls it a projectile.  It is always called a bullet.  Unless you reload ammo you will have no need to buy or own just bullets.


The case is what holds the powder, primer, and bullet all in one cartridge.  This case is called brass.  As most are made of brass. Although in recent years they are made of steel, aluminum, or other materials.  In fact the first cases where made of paper, and used for muzzle loaded rifles.  The end of the case was ripped off and powder was dumped down the barrel then the case with the ball was rammed in on top of the powder.

Sight Line

The sight line is an imaginary line drawn from you eye through you sights to the target.  If you are using iron sights it would include the rear and front sights being lined up with your eyes line of sight and the target.  If you’re using a scope it’s from your eye through the cross hairs to the target.  This line is always a perfect straight line.  No matter if the bullet hits the same spot as the sight line or not.  The sight line is laser beam straight.  This is why you must sight in your rifle to match the sight line to the bullets trajectory.

Bore Line

The Bore line is a straight line coming out of the bore of a gun.  This is the line many people use to bore sight a scope.  Using a laser from the gun bore.

It’s good to know that the bore line and sight line are not parallel.  If they where even at point blank range your bullet would always hit low.  Since the sight line is above the bore line from the start.  This of course is due to your sights being mounted on top of your gun bore.

Also do to bullet ballistics using the bore line to sight in a scope is not perfect.  However, we will get to that later.

 Bullet Ballistics

MOA (minutes of angle)

Minute of Angle or better know as MOA.  MOA is an angular measurement of 1/60th of a degree.

Think of MOA as a circular cone that extends out from your gun barrel.  This cone is where every bullet will hit if no outside forces act on it.  This means if you locked your firearm into a holding device and fired it multiple times all the bullets would hit in this circle.  1 MOA is equal to basically a 1″ inch circle at 100 yards away from the gun.

It’s called a cone because the farther the distance from the gun the larger the circle becomes.  There is a lot of complicated math that goes into this.  To simplify it.  For every 100 yards you add 1” to the diameter of the circle.

An example would be all your shots fall in a 1” circle at 100 yards. This is 1 MOA.  If shoot at a target 200 yards away, they will be within a 2” circle.  This is still called 1 MOA.  At 300 yards, they be within a 3” circle, still 1 MOA.   You get the picture.  They same holds true for any MOA.  1/2 MOA equals 1-1/2″ group at 200 yards.  This is the very simple explanation.


A click is one adjustment of your sight.  Either for a scope or irons sights.  Every sight will be different as to what one click equals in distance.  As an example many scopes will tell you that each click of the adjuster is equal to 1/4 MOA or 1/4″ of movement to the sight at 100 yards away.  This is where you have to understand what MOA is so you can understand what a click is.

Each scope or sight is different. You’ll have to look at yours to find what a click is equal to.  The simple definition of a click is one adjustment in your chosen direction of the sight line.

What is Bullet Ballistics

Bullet Ballistics is the path a bullet travels after it leaves the barrel of the gun.  Many times it’s a term that is interchanged with bullet trajectory, or bullet path.  It works to interchange these terms even though there are differences in the terms.

Bullet trajectory is the path of the bullet, effected by all the things that make up the ballistics of the bullet.  When most people talk about bullet trajectory they aren’t concerned over things like the amount of force a bullet has when it hits its target.

There is a myth that people generally believe when it comes to bullet ballistics.  This needs to be addressed before we go much further.  Bullets do not rise when they come out of the barrel of a gun.  The bullet path is always a downward arc towards the earth. Starting flat and getting more round the farther the bullet gets from the barrel.

The reason for many people believing that bullets go up before they start to drop is the difference between bore line and sight line.  The sight line is straight to the target.  Where as the bore line must be pointed upwards towards the sight line.  Just like when you throw a ball it goes up then back down in an arc because you know to throw the ball upwards.  A bullet is shot the same way.

Bullet Drop

Bullet drop is the how fast the bullet hits the ground verses how far it travels from the gun.  Of course this isn’t the standard definition.  Usually when someone talks about bullet drop or bullet drop calculator they are referring to how low the bullet will hit compared to where the sights where aimed.  This is based off the distance the target is compared to the distance the rifle was sighted in for.

Which means if the bullet drop is 10″ at 200 yards and the gun is sighted in to be on target at 100 yards the bullet has fallen 10″ in 100 yards.  Thus the bullet drop for that caliber on that gun would be 10″ at 200 yards.  Of course if you sighted the rifle in at 200 yards on that gun the bullet would then be considered 0″ at 200 yards but it may be 24″ at 300 yards even zeroed at 200 yards.  The farther from the gun a bullet travels the more bullet drop you will experience.

Ballistic Coefficient

ballistic coefficientBallistic Coefficient is a matter of aerodynamics.  There is a lot of complicated math to figure out the ballistic coefficient of a bullet, and you can see it all here.  That’s if you are a math guy.  For those who aren’t math people.

The important thing to understand is that the higher BC the farther the bullet will travel.  As long as velocity is equal to bullets of lower BC. This is independent of caliber and weight since both size and weight is taken in to consideration in the formulas that determine BC.

If you take two bullets in different calibers that have the same BC (ballistic coefficient) and fire them at the same velocity they will have the exact same trajectory.

Generally bullets that are heavy for caliper with pointed tips and boat tails have a higher BC.  Think about it like throwing a heavy football versus throwing a plastic baseball.  Your arm will try to throw them both at the same speed but the football will travel a lot farther.  The football has a higher BC than the plastic baseball.

Sectional Density

The ability of a bullet to penetrate is sectional density.  Low sectional density is like hitting a brick wall with a Prius.  Where as high sectional density is hitting the same wall with a Kenworth.  Which one is going through the wall?  Better yet which one is going to keep going, after going through the wall?

Sectional Density is the bullets weight in pounds divided by the square of it’s diameter.  This means the shape of the bullet doesn’t matter when you look at SD.  It would be very easy to have a bullet with a low BC and a high SD.  Think shotgun slug.  What really effects the SD of a bullet is the length.  Because a longer bullet will be heavier for caliber, it’s sectional density will be higher.

Bullet Expansion

Bullet ExpansionBullet expansion is how much a bullet changes shape with it hits a target.  The more a bullet expands the more energy it transfers to it’s target.  Putting a small hole through an animal can kill it if you hit vital organs.  A small entry hole and large exit wound is better.  Transferring all the bullets energy to the animal.  Will drop it in it’s tracks.

Imagine hitting someone with metal rod or a 12″ wide board.  Which is going to push them over?  The problem is it’s hard to swing a 12″ wide board as quick as the rod.  Thus if the rod flattened out the second you hit the person you’d get the best of both worlds.  This is bullet expansion.  Pointed bullets that fly well expand when they hit their target to release more energy into the target.


There is a lot that goes into understanding bullet ballistics.  In fact this post barely touched the surface.  But with even a slight understanding of sight line and bore line you can begin to understand why you can sight your rifle in at 2 different distances and it be dead on.  It will make sense when you’re told to mount your scope as close to the barrel of a gun as possible.

As you start to understand Ballistic Coefficient and Sectional density you will understand why there are so many different calibers.  With different bullet weights for each for the same caliber.  If you do more research you can spend hours figuring out the exact path of each bullet fired from you rifle based on shape, size, weight and velocity of you bullet.  Although most people won’t get that technical.  It is nice to be able compare the numbers on the side of an ammunition box and what they mean.  Better yet when you start reloading ammo it’s good be able to pick the right bullets for the right job.  Unfortunately with anything gun related the more you learn the more you realize how little you know.  So have fun learning more.

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