The Firearm Guide: How do bullets work?

I know this is a popular question, so I'll do my best to break it down as simple as possible. At the end of the day, the science behind it really isn't that difficult.

Lets see if we can load you up with some knowledge real fast-like.

Bullets, or projectiles, are the objects that are being fired out of a firearm. Just like I discussed on my article about firearm terminology, the cartridge inserted into a firearm is comprised of about 4-5 things:

1. A projectile, or bullet.

2. The cartridge casing itself, which houses everything else.

3. The explosive reactive propellant, like gunpowder.

4. The rim, which is where the primer sits, if applicable, as well as allows the cartridge to properly seat in the chamber and be extracted as noted in the guide about the cycles of function.

5. The primer, which in most rounds is used to activate the propellant. Some smaller rounds, like .22, don't need a primer and are activated just by striking their thin rims.

To make this easier to follow, and to prevent myself from going on any rants, we'll break this down into multiple steps:

1. Igniting the propellant

2. Gas pressurization of the chamber

3. Projectile Movement

4. Projectile exit

Igniting the propellant

The first step in this process is the activation of the cartridge propellant. This propellant is incredibly flammable, and this action happens almost instantaneously.

This is important because any uneven or prolonged burning of propellant means that the cartridge will move forward while some propellant is still burning, and this means less or uneven pressure will be behind that projectile. This means slower projectile, and could cause the projectile to shoot inaccurately.

Temperature also affects this. For example, a hotter round will burn quicker and create pressure faster than a cooler round. This means that your velocity isn't always the same, and might explain some difference in your shots. With smaller munitions though, this is relatively negligible.

As it burns up, it creates a gas. This gas in turn creates that pressure needed to force the projectile forward down the barrel.

This is the start of the process.

Gas pressurization of the chamber

Imagine blowing up a balloon, or closing your mouth and trying to blow outwards. You're creating a force with your air, which presses against the inside of the balloon walls or the inside of your cheeks - but it has nowhere to go. So, anything not being able to withstand that force is going to adjust as much as it can until it's not physically able too. Like a balloon expanding until it pops, or the fact your cheeks don't become eight feet big in circumference if you keep blowing.

Now if you blow without keeping your mouth closed, it just leaves your body like normal.

I think this is pretty common and easily understandable science. This is how your firearm works as well.

Think about how a round is seated inside of the chamber of a firearm.

Are you starting to get the idea?

The top, rear, and bottom of the cartridge are completely sealed off. This means that any pressure in this area is going to push against those walls, but won't be able to go anywhere.

This means that the pressure is going to go where it has the ability too - can you guess where that is?

Yep - down the open barrel, just like when you blow out of your mouth.

Lets look at this image again:

As you can see, the projectile, which we know is a separate entity of the cartridge case, acts as that fourth wall to keep the pressure trapped. Unlike the top, rear, and bottom of the cartridge though, the projectile will dislodge from its position once enough pressure has formed behind it.

The pressure does two things. One, it forces the projectile down the barrel. Two, it forces the bolt or striker backwards after it unlocks, allowing it to cock. Check out the cycles of function article for more insight into this.

Lets to move onto the next phase - movement.

Projectile Movement

Now that we have the pressure needed behind the projectile, it's time for it to start moving down the barrel. The barrel, as said before, is responsible for guiding the projectile straight as well as keeping the pressures behind the projectile evenly distributed.

It's important to keep your barrel free of debris and properly maintained. In a process that involves high pressure like this, blocking the end of the barrel doesn't just block the projectile - it blocks all the pressure behind it. If the tensile strength of the barrel or any of the components keeping that pressure contained is weaker than the pressure itself, that's where you get catastrophic malfunctions where the firearm explodes.

In similar concept, if the barrel is bent or damaged, it could either slightly redirect the projectile, allow the projectile to impact and further damage the integrity of the barrel, or cause in-bore balloting, which is when the projectile starts bouncing around the barrel as it moves - decreasing its accuracy.

The barrel is also responsible for causing the projectile to spin, which allows it to mitigate a lot of the aerodynamic forces that affect it once it exits the barrel.

Now, lets do exactly that - exit the barrel.

I won't be going too in-depth about rifled and smoothbore barrels here, but I'm sure I'll have an article about it soon enough.

Projectile Exit

Now that the projectile is exiting the barrel, there's a few things happening here.

First, all that pressure that was behind the projectile is going to come out of the barrel behind the projectile. It's going to create a sort of pocket for the projectile as well, and this prevents the projectile from being affected by any aerodynamic forces straight out of the barrel. Think of it like a shielding of pressure.

This evacuation of the pressure is also going to slightly affect the yaw of the projectile, or the angle it's aiming. This is usually a bigger case for weapons such as tank rounds, but this is also why it's important that the pressure is equalized behind the projectile. Any uneven pressure on one side might redirect the projectile slightly. Again, not too big a deal for smaller munitions.

If you want a visual for this, check out this Mythbusters video of a slow-motion firing.

Second, once this projectile exits that pressure pocket, it has to deal with aerodynamic forces. I know I've said this a few times, but what does that actually mean? Well, you have stuff like air pressure, wind, rain, etc. This is the natural, uncontrollable forces that can slow or redirect the projectile.

I mentioned spinning. Spinning the round allows us to fight back most of these forces by letting the round basically brush them off a bit, while also keeping the projectile straightened.

Think about how you throw a football. If you just lob it, chances are it will start spiraling uncontrollably.

Finally, you have the impact of the projectile. If everything worked properly, and you didn't have any issues with those pesky aerodynamic forces, then your round should impact relatively where you wanted it too.

There's a lot that goes into this, but overall I think it's a relatively simple concept that anyone, regardless of your knowledge of science and the like, can understand. At the end of the day, all we're doing is creating an explosive amount of pressure behind a moving projectile and aiming it with the rifle.

I hope you learned a lot, and as always keep checking us out for more content.