Eli5 : What are the dimensions mentioned in ammunitions? And how are they different from each other and what makes each one of them unique?


In most movies and video games I have observed people mentioning ammo type and capacity such as, 5.56, 7.76, 9mm, 0.50 calibrate, .45 ACP.

What are these ammo type ?

Edit1: 0.50 Calibre, my mistake!

In: Engineering

Those numbers are the caliber, which is a measure of the diameter of the gun barrel and the ammunition that fits it. The larger the number, the larger the bullet, which generally makes it more powerful. But keep in mind that calibers that are less than 1 (e.g. .50 and .45) are measured in inches instead of millimeters, so .50 is equivalent to about 12.7mm.

Also the higher the caliber the greater the penetration power. For example a .50 cal round will go through an engine block, while a 5.56 probably won’t go through a human body. The different calibers of round are for different things, different targets. You might not want your bullet to go through a person because it’d cause less damage, therefore you’d go with the 5.56 or the like.

5.56, 7.62. Etc are all measurements of the bullet width. If you have a box of ammo it’ll say, for example, 7.62 X 45, that is saying the bullet width is 7.62 mm and the case length (the brass that holds the gunpowder) is .45 mm long

There are lots and lots and lots of different cartridges for guns.

The history and reasons for which, will take far too long to go into. But basically, the numbers that bullets get is to do with their size.

There’s two bunches of sizes of bullets, there’s metric (in mm and grams) and imperial (in calibre which is fractions of an inch, and in grains)

there’s also two TYPES of ammo, generally. Rifle rounds, and pistol rounds.

Rifle rounds aren’t actually much bigger bullets than pistol bullets. But they have much much bigger cases, with way more space for gunpowder, which means they fly FASTER.

.50AE and .50BMG are good examples. One is a pistol round and one is a rifle round. They are the same thickness (half an inch), but one goes way way way faster. Google for images for a look.

But yeah, they’re just different sizes and shapes and fly differently. – there’s loads of resources online if you want to get deeper into it, but you just learn the different main types and then it all makes sense.

Generally these numbers are roughly the bullet diameter either in metric or in hundreths of an inch. But they are nominal numbers, which are not always the actual dimension.

In NATO metric designation the numbers are *nominal* *bullet diameter x case length.* So 9×19 is a 9mm thick round stuck in a 19mm long case, 5.56×45 is a 5.56 milimeter thick round stuck in a 45mm long case. American round tend to be named by the bullet diameter in hundreths of an inch, so .40 is not 40mm as some journalists seem to think, it’s 0.40 inch which is roughly 10mm. Older black powder rounds tend to have a dash in their designation, like 45-70. In that case the first number is caliber in hundreths of an inch and the second number is how much black powder in grains the cartridge holds

The problem is that the number is very rarely exact because there is no universally accepted naming convention and different ways to measure plus marketing considerations come into play. So is the number you are looking at the diameter of the unfired bullet? Is it actually the caliber (barrels have lands and grooves in them, caliber is the barrel diameter measured land to land). Is it the barrel diameter groove to groove? Is it an arbitrary number selected to make the round stand out? For example 38 Special is the same diamter as 357 Magnum. You can even fire 38 Special out of 357 magnum revolvers. 44 magnum is actually .43 inch in diameter. 300 Winchester Magnum is the same diameter as 308 Winchester. 9mm Luger and 9mm Browning which is also called 380 ACP are also the same diameter, but it’s not 0.380 inch, it’s 0.355 inch and to make it even worse, 9mm Makarov is not actually 9mm, it’s 9.27mm.

These are metric sizes:

5.56 – The bullet is 5.56mm in diameter. This is short for the full name, 5.56×45, the 45 meaning the case of the bullet is 45mm long. This is similar to the civilian .223 Remington, very slight differences but not in outer dimension.

7.76 – You probably mean 7.62×51, same meaning as above.

Neither of the above necessarily mean “assault rifle.” They were popularized by military rifles, but they are also commonly used with civilian rifles in the US. The 5.56/223 remains an excellent varmint cartridge due to its heritage (below), and it is the favorite for shooting feral hogs. The 7.62 is the same diameter as the .308 Winchester and the .30-06 Springfield, both of which are very popular hunting rounds In fact, all three of the .30 rounds have nearly the same power/performance/ballistics.

First confusing bit for you, much to follow: The .30-06 is .308 but the caliber is rounded down, and the -06 designates the year it was adopted by the military.

9mm – Also called 9mm Luger, 9mm Parabellum, or 9×19. numbers have same dimensional meaning as above.

Now we get into inches:

.50 calibrate – .50 caliber is .50 inches in diameter. You most commonly hear this in relation to .50 BMG (Browning Machine Gun), which is technically known by the metric 12.7×99 (notice how much bigger those numbers are than the above). It was originally designed for that machine gun, but its popular civilian use is in bolt-action or single-shot rifles.

45 ACP – This is the .45 Automatic Colt Pistol, aka .45 Auto, because that’s what it was developed for. It’s .45 caliber, or by metric 11.43x23mm.

Now here’s where it gets fun. Many of these diameter measurements are inaccurate for various historical reasons. The .223 Remington is based off the .222 Remington, a varmint round (foxes, coyotes, etc.) designed in 1950. They just stretched the case 2mm and moved the shoulder forward to increase powder volume a bit to get a few percent more velocity. They named it .223 to avoid confusion. And they’re both actually .224 diameter.

Very old cartridges often had a paper patch around the bullet so the barrel was bigger than the actual bullet. So you have the .45-70 being actually .458 in diameter, and today we use .458 bullets because we don’t use a paper patch. But it gets more confusing! That 70 does not refer to case length, but the standard charge of black powder the cartridge was designed for. This caliber-powder was the common naming convention for black powder cartridges in the 1800s.

The .30-30 is a strange exception. Originally called the .30 WCF (Winchester Center Fire), a competing maker called it the .30-30 to not use the competition’s name, with the second 30 denoting the amount of smokeless powder, using the black powder naming conventions of the day.

Then there’s the .44 Magnum, actually .429 in diameter because its grandparent cartridge (it’s based off a cartridge based off that) had the bullet diameter shrunk slightly for lubrication reasons upon request from the Russians who had commissioned it (this is in the Tsar days).

The .38 Special gets even more strange. It’s really .357 but just say the history way down involves a .38 caliber bullet in a black powder revolver, and the name stuck although the diameter didn’t. The .357 Magnum is just a .38 Special with a slightly longer case and it’s named with the correct diameter. FYI, the case is not longer to make room for more powder, but so that people couldn’t put a much higher pressure .357 cartridge in a revolver only designed for the lower pressures of the .38 Special.

Calibre is measured in inches traditionally but a lot of cartridges (bullet plus brass, primer, and smokeless powder, ie the bit you put into the gun) have metric designations. For example, 5.56x45mm is NATO’s go-to intermediate cartridge and came from the .223 Remington, and 7.62x51mm is NATO’s most common battle rifle cartridge and evolved from .30-06 (thirty-aught-six) Springfield. There’s subtle differences as the military cartridges were developed for man-sized targets and so they aren’t 1-to-1 copies of their sporting ancestors today.

9×19 Parabellum is a common pistol cartridge, as is .45 ACP (also called .45 Auto or .45 Browning, but is not to be confused with .45 Smith & Wesson or .45 Colt which is weird because the “C” in .45 ACP does stand for Colt).

.50 doesn’t really mean anything on its own, however it’s commonly applied to .50 BMG (Browning Machine Gun) which is the common NATO heavy machine gun cartridge used by the M2 Browning, however there’s also the 12.7x108mm used by former Soviet countries which is also called a .50 occasionally. .50 has become a byword for any similarly sized cartridge and doesn’t actually refer to a single cartridge.

Simply put, they’re different brands for different purposes.

ELI a bit older than 5:

Now it’s important to note that what cartridge is used in what firearm and the dimensions of the cartridge depend on many different factors. A lever action firearm chambered in .357 S&W Magnum will happily fire a .38 S&W Special, even though the two cartridges clearly have different dimensions (the .38 casing is slightly longer and wider, however because a lever action firearm is cycled manually rather than gas blowback a longer case doesn’t cause issues with ejection, and both cartridges are actually .357 of an inch in diameter despite having different names–the .38 actually refers to the diameter of the casing, so don’t go assuming that the number in a cartridge’s name directly refers to the diameter of the bullet, because it often doesn’t.

Within a single cartridge family you can have a lot of different types. Let’s stick to 5.56x45mm because it is by far the most common cartridge most people will experience, either in real life of through media. Now, the 5.56x45mm cartridge used by the US Military is called M855 and this has some very specific features: the bullet itself has a steel penetrator on top of a lead core surrounded by a copper jacket. The steel penetrator is a sharp point (painted green for easy visual identification when loading a magazine, AKA “bombing in”) to puncture armour and cover, the lead core adds mass (lead is very dense giving it more hitting power) and is malleable enough to deform when there is a sudden change of velocity (ie hitting something soft and wet–hollow point ammunition has the steel penetrator removed and the lead core is exposed, causing even more deformation). This deformation helps creates a cavity within the target. The copper jacketing is because a steel bullet is too hard, and will damage the barrel of the firearm (which has twisted groves in it called rifling to give the bullet a spiral spin–next time you throw a dart or a ball put a bit of a spin on it and see just how much this helps), so a softer metal is used instead so that is eroded rather than either the steel penetrator or the barrel. Because there’s different metals used this changes the mass of the bullet (measured in gr for grains, 1gr is 0.064 grammes or 0.002 ounces), so when you buy cartridges you have to consider how many grains the bullet weighs and what it’s composition is, because it could be the difference between eating some tasty venison or combing the woods for deer bolognese, and using military surplus M855 is a guaranteed way to get yourself kicked off a shooting range because those targets are expensive and the owner doesn’t want them blown apart too quickly.

Another consideration is the propellant. Almost all cartridges these days are smokeless powder which is smokeless (funnily enough) and it contains its oxide for combustion (and thus propulsion). Contrary to popular belief you can fire a gun in space because the oxide is already chemically present within the cartridge. Older cartridges use a material called cordite which looks like toffee and is still used in most fireworks. Firearm collectors often struggle to get original cartridges in rare calibres in cordite because it doesn’t keep well (I’ve fired .303 British with 90 year old cordite propellant and it was not a good experience). Black powder is totally obsolete outside of collecting and practical shooting sports if you’re LARPing as a cowboy. Propellant is also measured in grains and again that can change between different brands of cartridge for different purposes but in the same calibre.

That’s half of what makes a firearm work, the other half is the firearm itself. Barrel length, rifling twist ratio, barrel composition and construction, and many other variables impact how a bullet will fly. You’ll see firearms firing the same calibre with different barrel lengths because there is a tradeoff between velocity, impact force, and MOA (minute of angle, AKA accuracy). If the barrel is too short the bullet won’t pick up enough velocity to fly fast and deliver full force of impact, but if the barrel is too long then you introduce friction and the bullet will start to decelerate before it has left the barrel. For 5.56x45mm M855 NATO a 20″ long barrel is regarded as being the sweet spot, but there are many types of bullet and propellants that are designed for specific barrel lengths, and if you use your firearm for plinking (close target shooting) or pest control other barrel lengths like 14.5″ (carbine length) is also popular because high velocity and MOA actually isn’t necessary in a close environment and the shorter barrel length will be easier to store, carry, and manoeuvre. As the majority of shooting occurs at below 600 metre ranges carbine length rifles tend to be the norm, for both sporting and combat.