Bullet Caliber and Ammo Guide: A Technical Course On All Things Ammo

bullet caliber guide

The subject of bullet caliber is one of the more confusing matters for gun enthusiasts. Caliber and ammo aren’t just confusing for newer gun owners, but also for long-time gun owners.

You can see that in all the Internet debates that commonly are inspired by caliber and ammo.

What does caliber mean? What bullet caliber is best for self-defense? Which ammo should I buy for this or that?

I’m going to dive into all of this and so much more. After reading my bullet caliber and ammo guide, you’ll be an ammunition expert.

My Definitive Bullet Caliber / Ammo Guide (Understanding Ammunition)

Ammo is important (that much is obvious). Choosing the wrong ammo can decrease your safety and security. It can harm a firearm. It can get you into legal trouble.

But here’s the thing…

Ammo, bullet caliber, its all really simple.

Let me show you why it seems incredibly complicated.

Bullet Caliber Is Size

The caliber of the bullet is simply the width of the bullet. In actuality, it is the width of the cartridge. The bullet, technically, is inside the cartridge. I realize I’m using the term in a misrepresented way, but for the sake of simplicity and what people typically search, I’m staying that course.

So let’s keep this simple and logical for a second.

The width of the bullet determines which size gun barrel it can go into. Therefore, the bullet caliber (AKA width size), determines whether or not it can be fired through a specific gun.

Simple concept – A bullet with a width larger than the circumference of a gun barrel will not fit through that barrel.

The barrel of the gun will be slightly larger than the caliber (width size) of the ammunition.

Let’s use a working example.

This is my trusty CZ PC-10 C. It is calibrated for 9mm ammo.

cz pc-10 cThis is a


Here is a 9mm hollowpoint (don’t worry, we will get into more technical aspects later).

bullet caliber 9mm

The 9mm round is sized to fit into the CZ PC-10 C’s barrel, therefore, it can fire through it.

My CZ PC-10 C probably has a barrel circumference of 9.002mm, or something like that. It would be a shade over the ammo’s size, for obvious purposes.

Now, to really drive this concept home, here’s my Glock 43. Its also calibrated for 9mm ammo.

Glock 43 9mm

Notice anything odd? My Glock 43 is a lot smaller than my CZ PC-10 C. BUT THEY TAKE THE EXACT SAME AMMO! That’s because the barrel size is all that factors into the ammo, not the overall size of the gun. People tend to get this idea confused, which often leads them do believe that the size of the gun equates to its potency.

Here are both my handguns, the Glock and CZ, and the 9 mm round that’s used in both. Both of these guns have the exact same lethal impact when fired.

bullet caliber guide

That didn’t feel too complicated, did it?

So then…

Why Is Bullet Caliber So Confusing?

You aren’t the only one who finds bullet caliber confusing. But why the heck is it confusing?

Because two metric systems are at play.

There is the Metric system, which is using millimeters. And there is the American system, which uses inches.

Here’s what I mean…

The above example I used with a 9mm round made all kinds of sense. But now, you are probably thinking…

“It makes sense that a 9mm round is 9 millimeters in diameter, but what about .22 or .223?”

The .22 round is measured in inches. So, it’s .22 inches.

There Is No Set Standard

Here’s the kicker, there is no set standard for metrics governing ammo.

Some ammo is measured in inches, some in millimeters, some are measured in inches or millimeters + the length of the cartridge.

The point?

Being confused is OK.

Take a breath. Its a big mess of numbers. If there was a standardized, uninformed way of assessing caliber, it would be better for all of us. But that’s never going to happen.

Most likely if you see a (.) preceding the round, that means it is measured in inches. Most likely sans the (.), and you have millimeters. If you see a number following a hyphen, that either means the length of the cartridge or sometimes grain (relax, we are getting to all of that).


  • So now, we understand that bullet caliber is the diameter of the ammo cartridge. That size determines whether that ammo round can be safely fired through a gun’s barrel.
  • Some people refer to a gun when they say caliber, others refer to ammo (we are talking ammo).
  • Bullet caliber is super confusing because the metric system uses two systems.

Let’s look at popular ammo types now and how we choose what’s right.

Centerfire or Rimfire

When a bullet is loaded into a chamber of a gun, pulling the trigger should enact the firing pin. This pin will pop the back of the bullet’s cartridge, thereby setting off an explosion of gunpowder.

Pressure, pressure, pressure…

The pressure will force the bullet to fly out of the barrel. The rest of that pressure is often utilized in popping the gun’s slide back, thereby, loading up another round (this is typical in semi-automatics).

So, the firing pin strikes either a rimfire or centerfire.

What’s the difference?

Let me show you visually…

rimfire primer vs centerfire primer

Yep, it is that simple. As the ilustration shows, its all about where the primer strikes. You can easily tell if a bullet is rimfire vs. centerfire by looking at the back of the ammo cartridge.

Choosing Ammo

We are beyond the barrel now. We understand how to choose ammo for our gun. We understand that caliber is the width of the ammo.

But how do we really, really, choose our ammo?

There are two considerations:

  • We choose ammo within our caliber
  • We choose ammo based on our choice in firearm

Before you buy a firearm, you need to decide what ammo you need.

In other words, the bullet caliber should determine the firearm you buy (not the other way around).

The .22 rifle sure is pretty, but don’t bet on it if you are charged at by a bear in Yellowstone.

No one wants to be tasked with shooting a bird from the sky with a 9mm.

There are several ammo purposes to consider:

  • Hunting: The type of animal you hunt will be meaningful here. There are state game laws that govern this arena, as well.
  • Self-defense
  • SHTF/Prepper (I have an entire section below called “prepper ammo concerns” that will help here)

Now, once you decide the intended purpose of your ammo, you can move through some of the most popular types of ammo and decide what works for you.

Ammo Grains

The grain of the ammo is a representation fo the weight and density of the ammo.

This is relevant to the power and potency of the ammo.

If you compare a mallet to a hammer, the mallet has more weight and more force when it is dropped down on anything than the hammer. But, you can throw the hammer a lot further than the mallet.

This is the science of mass and energy at play and that’s what ammo grain is all about. The way a bullet soars through the air impacts a target and does (or doesn’t) divert when it encounters other objects (think plants) is effected by the ammo’s grain.

One ounce is 437.5 grains. Ammo grain refers to the mass of the bullet, this doesn’t include the cartridge that houses the length of space for gunpowder.

Here are some common ammo grains

  • 9mm – 115 to 147 grain
  • .380 ACP – up to and around 95 grain (never more than 100)
  • .40 ACP – 165 to 180 grain
  • .45 ACP – 185 to 230 grain

The amount of grain in one specific version of ammo can affect its price, its potency, and how lethal it is.

So how does this work?

Speed / Impact

A heavier bullet flies slower, but it impacts its target with greater destruction. A .45 ACP is larger than a 9mm. The .45 ACP will do more damage to a close-range target. The 9mm, however, flies faster.

The 9mm is smaller and lighter than the .45 ACP, so it has more muzzle velocity.

Ammo Cartridges Broken Down

Ammo cartridges are broken down by the following:

The bullet – this is what flies out of the gun’s barrel after the cartridge explodes.

The case – Usually, the case that houses the bullet and gunpowder is made of aluminum, brass, or nickel. I break down the aluminum vs brass debate way down the page.

The primer – Whether it be centerfire or rimfire, striking the primer is what causes the gunpowder to explode.

Gunpowder – The gunpowder is what explodes after the striker hits the primer.

Types of Ammo

I am only going to list out popular ammo.


The 22 long rifle is the world’s most popular round. Its light, weighing in around 30 to 40 grains.

Because the .22 is a smaller bullet caliber, it packs much less punch than a lot of other ammunition does. You won’t feel much in the way of recoil if any. The sound is lower.

This is a less intimidating round than most other ammo, therefore, it’s really good for beginners.

Its great for shooting a squirrel with, or plinking, or learning to use sights.

It’s not a great option for self-defense because it simply doesn’t pack that much of a punch. Yes, accuracy in self-defense shooting is a primary component, but so is wound-size and knock down power. The .22 round just doesn’t have much in the way of the latter two components.

.25 ACP

.25 acp

Low power and intended for ultra-short ranges, this light round isn’t overly popular in modern handguns.

The .25 ACP cost more than a .22 LR and isn’t considered much more powerful. Oddly, the .25 ACP is more powerful than a .22 LR, but only when fired from handguns. So for personal defense, the .25 ACP is a bit better than the .22 LR.

Overall, this isn’t a great ammo option for a variety of reasons.

.380 ACP

*up to 95 grain

The .380 ACP is one of the most popular self-defense rounds there is. Its weight is anywhere from 85 grain to 95 grain.

The .380 ACP is a controversial round as it is often compared to the superior 9mm. But hey, all things must be considered…

The .380 ACP is commonly put in smaller, extremely concealable guns. The round itself is lethal, particularly the more accurate the shooter is.


*115 to 147 grain

This is my absolute favorite/preferred handgun ammo. The 9mm is a popular round for self-defense. It is used worldwide by militaries and police forces. It’s lethal and accurate at short and moderate distances.

In my opinion, the 9mm is a much better option than the .380 ACP. Its much more potent and many handguns, including the Glock 43, are extremely concealable.


*180 to 200 grain

Just like a 9mm, but one more millimeter. This is an underrated round for hunters and campers who need something a little more potent than a 9mm in the event they are threatened by a wild animal, but don’t want to carry a full .45 ACP round that’s much heavier and more robust.

The 10mm is overlooked often, but it shouldn’t be.

.40 S&W

Developed by both Smith & Wesson and Winchester, the .40 S&W is a lethal round.

The overall cartridge comparison is close to a 10mm. However, gun owners love to compare the .40 S&W to the more popular 9mm for the sake of starting Internet fights.

The .40 S&W is not as potent as the 10mm, but it is more potent than the 9mm.

.45 ACP

*185 to 230 grain

The .45 ACP is one of the most potent handgun loads there is. If it weren’t so heavy, I’d say it would be the best handgun caliber around (and hey, depending on your preferences, it might be).

The .45 ACP has severe knockdown power. It can help you defend even against a bear.

The only downsides are recoil, which can affect subsequent shots (one reason I prefer the 9mm). The other downside is weight. The .45 ACP is a big round and when you load 17 of those up in a magazine, your gun becomes pretty heavy.

In terms of recoil, I want to make a note that the Glock 21 manages .45 ACP recoil like a champ. If you are debating between a 9mm or .45 ACP, I’d suggest you test fire a Glock 21 (.45) and Glock 19 (9mm) side by side. Feel their weight, experience their recoil, and see how you feel shooting both. You might be surprised by how well the Glock 21 handles recoil.

.38 Special

The .38 Special has lost its luster over the years, but that doesn’t mean it isn’t still one of the most powerful ammo rounds one can find.

It’s inexpensive and potent and lethal. But modern semi-automatic handguns don’t typically take them (some do, its just rare). The .38 Special is fired most often from a revolver, something that feels ancient to most people today.

That said, I get it, Revolvers don’t offer the capacity that modern semi’s do.

.357 Magnum

Like the .38 Special, the .357 Magnum is most often used in revolvers. It packs a huge punch. Invented in 1934, the .357 Magnum also has massive recoil.

5.56 – .223

.223 bullet caliber

The 5.56 and .223 ammo conversation is one of the more confusing ones.

In fact, I’ve got an entire 5.56 vs. .223 ammo guide you can check out for a lot more information on the subject. The 5.56 is a NATO round, it has little more pressure than a .223 round, but overall, they are the same. A gun calibrated for .223 can’t fire a 5.56 round, but a gun calibrated for 5.56 can fire .223. The 5.56 will fire through the .223 calibrated firearm, but given time, it destroys your gun from the added pressure.

This, in my opinion, is the most versatile ammo on my list. I say this considering potency, price, availability, recoil management, accuracy, etc.

The 5.56 is not the preferred round to take down a deer with, but given its long-range accuracy, it most certainly can get the job done. In terms of self-defense, when used from the AR15 platform, the less consequential recoil means rapid fire shooting is easily accomplished.

The .223 round is the same caliber as the .22 LR, however, as you can see in my picture. the .223 is longer and houses more gunpowder. The top is pointed for penetration.

.308 – 7.62x51mm

The .308 is a popular hunting round. It’s more potent than a 5.56 round. It’s also more expensive.

The 7.62 signifies the NATO round. The .308 can be a heavier pressure load.

This can be a hunting round, or even a sniper round. The AR10 platform uses this ammo.

7.62x39mm – AK47 Ammo

Powerful and highly underrated. The 7.62x39mm was invented by the Soviets. It is famous for being used in the AK47.

People often confuse the .308 as being an AK47 round, but the AK47 wouldn’t work so well managing all of that recoil.

AK47s and AR15s both use lighter, but lethal, ammo, that doesn’t give an overload of recoil. This allows rapid fire with higher accuracy.

12 Gauge

12 gauge bullet caliber

Truly one of the most potent ammo types around. Now, 12 gauge ammo can come in various loads. There are waterfowl loads that have lots of pellets. These spread out and take in a broader target. Higher number waterfowl loads are great for killing ducks and geese but would be rather ineffective if used for deer or self-defense.

The mighty slug or buckshot, however, is like cannon fire. A slug fired from a 12 gauge shotgun is a single lead ball that can knock down a bear.

Versions of Ammo

Now that we know about the different types of ammo, let’s look at the types of ammo within each ammo type.

Wait, what?

OK, we know what a 9mm ammo is, but do we know the different types of 9mm ammo?

Most ammo has unique types that offer specific advantages.

Here are the different terms and types to know.

Lead Round Nose (LRN)

The lead round nose bullet is the most popular and most used bullet type. The lead round nose gets great penetration and cuts the air with relative ease.

The problem with LRN is that many places don’t allow lead rounds anymore due to potential contamination of the ground.

Wadcutter (WC)

Designed for use in revolvers, the Wadcutter is the shape of a cylinder. Its most closely associated with .357 Magnum and .38 Specials. They have excellent penetration and wound potential.

Semi Wad Cutter (SWC)

Semi Wad Cutter (SWC)
By DrHenley – Own work, CC BY 3.0,

A flat nosed bullet that’s good for target shooting (large paper holes left behind), but not a great self-defense or hunting round by today’s standards.

Semi-Jacketed (SJ)

Semi-Jacketed (SJ)

The soft lead point means that the Semi-jacketed ammo round expands on contact, which means more wound potential. However, the semi-jacketed round gets more penetration than a typical hollow point does.

Full Metal Jacket (FMJ)

full metal jacket
7.62x39mm full metal jacket ammo used in AK47.

A full metal jacket round is a soft lead point housed by harder metals. This is an extremely popular type of ammunition. Its known for its lightweight and ability to fly fast through the air.

Semi-Jacketed Hollow Point (SJHP)

bullet caliber 9mm

Most commonly known as the “hollow point” bullet, its purpose is to do extreme damage to any mass it encounters.

Jacketed Hollow Point (JHP)

The same concept as the semi-jacketed hollow point, the jacketed hollow point allows the bullet expansion plus deeper penetration.

Special (RCBD)

These are blended-metal bullets and known to be bad. I’m just putting them on this list so you know what they are, I’m not recommending them.

Prepper Ammo Concerns

When it comes to prepping, ammo is a big concern. You can have the highest quality guns ever made, but without ammo, they are rendered useless.

Because of this, you need to consider the caliber of your prepper gun, or SHTF gun, before you consider the gun itself. Sounds weird, I know, but its the logical approach.

If you are just buying a gun for fun, or for EDC self-defense, this section won’t mean as much to you. But if you are buying a gun for SHTF, or doomsday scenarios, you must consider the caliber of the gun first.

The ammo that goes in the gun will influence how overall prepped you will be.

When it comes to prepper ammo, consider the following.


First and foremost, how available is the ammo today?

If you buy a SHTF gun today, your plan is likely to stockpile ammo over time. Well, how easy will that task be?

There have been cases in the past where certain ammo was difficult to find. Go to a WalMart, look and see what ammo they have consistently stocked. This can give you a really good idea.


What’s the knockdown power of the ammo?

A .22 round is readily available for cheap, but it has little knockdown power.

9mm, on the other hand, has great potency and its available.


Is the ammo a good short-range, knockdown type ammo?

Can you kill a deer or elk with the ammo?

Can you use it to defend yourself?

How long-range is the ammo accurate?

A shotgun slug is a lead ball that can absolutely annihilate anything in its path. But for how long-range? 100 Yards would be your max.

A 9mm round isn’t good for long range, but it does severe damage at 20 yards.


Straight up, how much will your ammo cost?

When it comes to prepper ammo, I’m a big believer in putting in a monthly order for ammo. That said, decide how much your budget is, look at the average price of the ammo for the gun your considering, then figure out how much ammo that equates to per month.

DO NOT PRICE RANGE AMMO (I have a section on range ammo below). If doomsday or an emergency comes, range ammo might not cut it. I’m not saying its worthless and you shouldn’t stock up with it (any ammo is better than no ammo), but make sure you price some decent quality rounds.

What Does All This Mean?

The best prepper ammo is 5.56 or .223. The military uses 5.56 as their standard NATO round. If SHTF happens, that ammo will be in circulation. This means that if you have an AR15, you will have a firearm that uses ammo that’s probably in circulation.

Furthermore, .223 ammo is cheap, readily available, effective in both short and long range, and deadly accurate when fired from the AR15 platform.

I’ve reviewed what I think is the best SHTF gun. Hint: it’s an AR15 made by Smith and Wesson. It takes both 5.56 and .223. If you do buy an AR15 for a prepper gun, make sure you buy one chambered for 5.56. A 5.56 gun can also fire .223. But a .223 can’t fire 5.56, which cuts down on ammo opportunities. Read 5.56 vs .223 ammo if this is confusing.

Now, the AR15 is a rifle. It’s versatile so you can defend your life with it, you can hunt with it, you can shoot 400 yards away accurately, you can carry it miles because it is light… but it is a rifle.


If you can, have more than one prepper gun. If you can only have one, go with the AR15.

You should still have a handgun that you can conceal. I’d recommend a Glock 19 or CZ PC-10 C. Smith and Wesson has some great options in this category. These take 9mm ammo. While 9mm ammo isn’t as cheap as .223, it’s not terribly expensive and it is readily available.

This gun would be for self-defense. You can pull it fast and it’s effective.

You could go with a .45, such as the Glock 21. The .45 caliber bullet has a lot more knockdown power than a 9mm. But fully loaded, the Glock 21 is heavier than a Glock 19. The .45 caliber ammo is pricier, as well.

Last but not least, I’d round off my prepper guns with a 12-gauge shotgun. Nothing has more knockdown power than buckshot or a slug coming out of a 12 gauge shotgun. You can take down a bear with it. You can easily kill a deer. It can blow open doors.

But, its heavy, the ammo is pricier, its got massive recoil so rapid fire is much more difficult. But as a third option, you can’t go wrong with a 12 gauge shotgun.

Recommended Prepper Gun List: AR15 / Handgun in 9mm or .45 / 12-gauge shotgun.

“Range Ammo”

If you are new to purchasing ammo, you have probably heard the term “range ammo” on a number of occasions.

So just what is range ammo?

Its ammo that is cheap to buy, allowing you to purchase more of it and practice shooting. It is also ammo that the range you are shooting at allows you to shoot. For example, many indoor ranges do not allow hollow point bullets or shotgun ammo that’s more than buckshot or slugs.

The term is used in places such as WalMart to let the cashier now that you are looking for cheap ammo that’s primary purpose will be to use at a range.

Range ammo isn’t something you want to use for self-defense loads or hunting loads. It’s less reliable and less impactful.

That said, range ammo can be dirty. If you fire a lot of range ammo, clean your gun afterward. Range ammo can also be less consistent in terms of target shots and groups. This is because the cheaper loads are often loaded inconsistently.

Some “range ammo” are reloads. I’m not a huge fan of reloads because with each reload, the cartridge tends to swell. I’ve had 9MM range ammo that was derived from releads misfire in my CZ PC-10 C. This is because the swollen cartridge fails to eject or allow the slide to pop all the way back in place.

Range ammo serves the purpose of allowing you to cheaply buy practice rounds. You need to practice shooting! But the ammo can run dirty, be much less accurate in target groups, and be unreliable.

If you have trouble with a particular brand of range ammo in a specific firearm, just stop using it.

Is Aluminum Cased Ammo Bad For Your Gun?

aluminum cased ammo
The bottom round is an aluminum cased .223 ammo round and the top is a brass cased .223 ammo round.

At some point, you are going to be told that aluminum cased ammo is bad for your gun. And you’re probably going to freak out because you’ve fired a ton of aluminum cased ammo for range purposes.

But let’s slow down for a minute and get our aluminum bearings.

Aluminum cased ammo is popular mainly because it is super cheap, making it an outstanding range ammo option. Additionally, its tough and super light. Its counterpart, brass cased ammo, is a lot more expensive.

For prepper ammo purposes, aluminum ammo stores wonderfully because it is corrosion resistant.

Now, if you are someone who chooses to reload ammo, then aluminum cased ammo just won’t work. While some people claim to reload aluminum cased ammo, it’s not advised to do so. This, of course, can affect cost and should be considered into the equation.

If you are using an automatic weapon, aluminum cased ammo might melt.

Brass ammo runs a lot cleaner than aluminum cased ammo does. Brass cased ammo seals off better, therefore, leaving your gun’s chamber less dirty.

Aluminum cased ammo notoriously gets jammed more often than brass cased ammo.

This is a tricky subject. I’ve fired thousands of rounds of aluminum cased ammo through my AR15, but I always clean it after. It will cost you less to do more, which is always a good thing when you want to shoot a lot.

Aluminum ammo should be treated as cheap range ammo. It isn’t for self-defense or hunting.


Now you understand how ammo works. You understand that ammo choice precedes firearm choice. You realize that different ammo types serve vastly unique purposes. Cost and availability of ammo can determine its value to a preppers list.

Ammo is a confusing subject even to avid gun enthusiasts, so don’t feel bad if you end up occasionally needing to ask more questions. It is normal. Ammo and bullet caliber info isn’t easy stuff.

You will often find that sales clerks at gun stores are just as confused over the subject of ammo, even if they pretend they aren’t.


Author: Jim Satney

PrepForThat’s Editor and lead writer for political, survival, and weather categories.

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