Please explain bullet sizes for guns (and other ammo jargon)

In a recent thread it was mentioned that an AR-15 rifle can shoot a .223 or a 5.56 or a 22LR rimfire up to 50 caliber round and .300 AAC Blackout and 6.8mm Remington SPC.

That is just one example and just one gun and I find it hard to keep track of what I am being told.

Can someone make this simple? .223 or 5.56 or 9mm or a 38 or a 45 or rimfire or 50 caliber and so on? I am sure I am not covering it all (full metal jacket comes to mind).

I assume there are a few measurements. Bullet mass, bullet diameter, how much powder is in the casing and so on but there seems more to it.

Thanks in advance.

Caliber is the dimensions of the bullet.
.223 or 50 caliber means the bullet (the part that flies through the air) is .223 inches or .50 inches in diameter.
5.56mm or 7.62mm is the diameter in millimeters.

rimfire means the firing pin strikes the rim of the casing (the part that holds the explosive charge) to set of the propellent charge. center fire means the firing pin strikes the center of the casing.

full metal jacket (or partially jacketed) refers to a hard metal (copper I think) casing that surrounds the softer lead bullet (the projectile). It prevents the bullet from mushrooming on impact to give it more penetration (for shooting through walls, vehicles or body armor,

They sometimes refer to the round length as well. For example:
5.56×45mm NATO is the standard round for an M16 (and most NATO rifles). You’ll note that the shell casing is much bigger than the .22LR used in an old 22. Even though the bullets are the same diameter.

Part of the reason for the confusion is that most AR-15s can shoot either 5.56 or .223 ammo. They are so close in size as to be interchangeable. Generally speaking a 5.56 will have more powder, so some rifles are chambered in (“chambered in” refers to what round it’s supposed to fire) .223 and could potentially be damaged by the more powerful 5.56. Others are chambered in 5.56 and effectively backward compatible with the less powerful .223.

Since .223 refers to the bullet diameter, this size is close enough to .22 LR that a .22LR bullet can be fired out of a .223 barrel, but the cartridge itself is smaller so you can’t fire the much less powerful .22 LR without modification to the gun. That modification is extremely easy and can be done in a few seconds with the right parts.

To get a standard 5.56 AR-15 to fire something other than these 3 rounds requires substantial changes to the weapon, replacing major parts such as the entire “upper” which is most of what you probably think of as “the gun” but legally in the US is just a piece of metal.

If it’s not confusing enough that diameters are sometimes in inches and sometimes in millimeters, and that the decimal is sometimes omitted, the ammo for some firearms such as shotguns are instead measured in “gauge”, which isn’t any sort of unit, and has smaller numbers for larger ammo. IIRC, the numbers originally referred to the number of pellets that would fit into… something, but nowadays it’s pretty much just a label.

For that matter, I suspect that that’s mostly true even when there is a unit: Most people with guns just know that there’s a kind of ammo called “.223” and a kind called “5.56” and so on, and know which kind or kinds each of their guns takes, but don’t know what the numbers actually mean.

I’m not sure it would be true to say MOST people don’t know that. Hell, I don’t have any guns and I know that.

There are some confusing terms - the .38 special, for instance, is actually the same width bullet as a .357 Magnum - but the general concept is pretty well understood by most shooters, even if the details sometimes are not.

Take a pound of lead and form it into equal size balls the size of the bore. That’s the gauge number. So 12 gauge would be 12 balls of that size (.73 inch) weigh a pound total. Higher the number, the smaller the balls.
Has nothing to do with the size of the pellets contained in the shell itself.

Here’s a fairly basic page that might provide the information you’re asking.

It covers pistol/handgun ammo, but also touches on rifle ammo.

If you have specific questions I’m sure we’ll have answers.

In case it wasn’t clear, by the way, my suspicion that most gun-owners just use the different sizes as labels wasn’t meant to imply that I think that gun owners, specifically, are unintelligent, incurious, uneducated, or the like. And I’m sure that a much higher proportion of gun owners than non-gun-owners would know those what the sizes mean. My suspicion is based on people in general being unintelligent, incurious, and uneducated.


.50 caliber is 12.7 mm. 7.62 mm is .308. :wink:

Yeah bullets are weird because they use both Imperial and Metric numbers pretty freely.

9mm, .38 Special, .380 ACP, and .357 Magnum are all the same bullet width (with a few minor MMs subtracted here or there)


I believe you (I truly do) but…


I believe you if you say it works. I am just amazed that a precision gun is that tolerant. Close enough works.

So…you can but you probably shouldn’t?

The way I’ve heard it is that you can safely fire .223 in a 5.56, but not the other way around.

Ultimately the consolidated answer to the OP’s question is that caliber is the diameter of the bullet. That can be measured in metric or US standard, or sometimes both. For example, .357 inches = 9mm, 7.62mm = .30, and so forth.

The metric method is generally written as bullet diameter x case length. So you’ll see things like 7.62x39 (AK-47 round), or 9x19 (the classic 9mm pistol round), or 7.62x51 (7.62 NATO). Sometimes there’s a further descriptor - like 9x18 Makarov. The US standard system is typically the diameter and the manufacturer or some other descriptor. So you have .308 Winchester, .375 H&H, .38 Special, .44 Magnum, etc…

Sometimes these measurements aren’t quite accurate. For example, the 9x18 Makarov is actually 9.27mm, instead of the exact 9mm of the 9x19 Parabellum. Or that .38 Special is actually .357 (which is also 9mm, FWIW).

Shotgun shells are their own thing- the “gauge” indicates the number of lead balls of the bore diameter that adds up to a pound. So a 12 gauge has a diameter of a lead ball 1/12 of a lb. They also come in lengths - 12 gauge shells come in 2.75", 3" and 3.5" lengths for example. So you might see a 3" 12 gauge shell. The shot (the small balls) in the shotgun is described by number- #9 shot is very small, and #1 is fairly large. Then you get BB size (just like a BB gun fires), and up to T sized shot. Then you get into buckshot, which is the same sort of idea, but different sizes, starting with No 4, which is the smallest, and all the way up to 000 buckshot, which is the largest.

Magnum generally means a higher pressure (more gunpowder) version of the base round. So you’ve got 38 special and 357 magnum, where the 357 is the same bullet, and the same case but just a bit longer so it can’t be fired in a 38 special gun. Or 44 Special and 44 magnum.

Rimfire is an older priming method, where the cartridge case is manufactured with a hollow rim, and primer compound is put in that hollow rim. That way, the firing pin just impacts the rim and fires the bullet. This is opposed to centerfire, where a separately manufactured primer is pressed into a little hollow in the center of the cartridge case. Rimfire is typically used in small-bore cartridges- specifically the .22 long rifle, 22 short, and 22 magnum cartridges these days.

You’ll see bullet weight as a secondary descriptor on many cartridges. It’s almost always measured in “grains”, one of which is roughly .065 grams. So you’ll see 124gn 9mm rounds, or 55gn .223 Remington rounds, etc…

Full Metal jacket, hollowpoint, softpoint, etc… are different types of bullets. Full metal jacket (FMJ) means that the entire bullet, save maybe the base is encased in a copper jacket. These typically don’t deform on impact, or at least not much. Hollowpoint is what it sounds like- a bullet with a definite cavity in the nose intended to cause the bullet to deform in a specific way. These are usually low-speed bullets primarily used in pistols. Softpoint is the same idea, but for high powered rifle rounds. At those speeds, they don’t need a cavity because a soft exposed lead point ahead of the copper jacket will cause those bullets to deform into a mushroom shape, just like the hollowpoints do at lower speed. In fact, IIRC hollowpoints at high speed will actually fragment more often than softpoints.

Powder volume is dependent on what sort of powder is in the case, and you can find that out from reloading manuals. It’s generally not important for buying ammunition.

On a not unrelated note, can anyone explain ammunition designated as “+P”? I’m guessing these refer to shells which have a higher powder charge than normal resulting in higher chamber pressures on firing. All other case and projectile dimensions and specifications are unchanged resulting in what my father would have called a “hot load”? Weapons intended for that cartridge can freely chamber both standard and “+P” rounds. If the weapon is not “+P” rated though, firing such a round will most likely result in damage to the weapon and injury to the shooter.

Is this more or less accurate?

I have a Colt 1911 chambered in Super .38 and I cannot find any non-"+P" shells for it. I also can’t seem to find a definitive answer as to whether or not my Colt is +P rated although the consensus seems to be that it is not. I would appreciate the input of knowledgeable Dopers on this issue.

Then there are the .30-30 Winchester (0.30 inches in diameter, with 30 grains of smokeless powder), and the .30-06 Springfield (0.30 inches in diameter, adopted by the U.S. Army in 1906). Still two of the most popular calibers for hunting rifles.

First DON’T shoot +P (or godforbid +P+) ammo out of an unrated pistol. DON’T.

Okay, so, +P / +P+ and the like are varieties of “overpressure ammo” with greatly increased pressure and thus energy and muzzle velocity.

+P after being unregulated for many years finally has a variety of official SAAMI standards for some calibers (generally about 10% more powerful) but +P+ is generally not regulated. Firearms will generally have no rating or unknown (just Don’t again), ‘limited’ +P (won’t fail dramatically but large volumes will damage the firearm over time), or are fully rated for +P.

I own several +P rated firearms but almost never use it - it’s expensive, and as a target shooter primarily, utterly unneeded. The only exception is my carry pistol (which I haven’ in years) - in that the expected results with +P combined with personal defensive ammo (normally jacketed hollow points) gives you some penetration with good ballistic performance.

[ aside for the thread jacketed hollow-points take the FMJ explained above, but the tip has a smallish (compared to some) hollow to allow easier feed in semi-auto pistols, better penetration than soft lead tip (or flat tip), and better terminal ballistics in a final shoot - as well as slightly fewer overpenetration issues than normal FMJ.]

Again, +P and it’s ilk is part of the more is better. Most of the time… I don’t think so. My (and many other home defense planners) think the biggest issue isn’t penetration, it’s OVERPENETRATION. I don’t want to shoot (at all TBH first) and have a round go astray across the street / through the wall / up and out. And even a 9mm will do this.

ETA - this is actually why a large segment of gun owners consider 5.56 of AR-15 fame to be terrible home defense weapons, sure, lots of rounds, lots more range than a pistol, but dear god 5.56 is a nightmare of overpenetration scenarios unless you’re defending a multi-acre range/farm all by yourself.

Thank you for pointing out it used/uses smokeless powder. Many people believe the ‘-30’ was for 30 grains of black powder.

AIUI, the ‘-30’ suffix was applied by Marlin to emulate the naming of black powder loads of the time.

Not quite; 9mm Parabellum (or Lugar or NATO) and .380 ACP (9x17 mm) are both 0.355 inches in diameter and .38 S&W Special and .357 Remington Magnum are both 0.357-0.358 inches in diameter, but the bullet weights are different depending on the loading, and of course the cases are different geometry preventing them from being interchangeable (save that a .38 Spl can generally be fired in a revolver chambered for .357 Magnum, but not the converse). The East Bloc 9x18 mm Marakov, on the other hand, actually has a bullet diameter of 9.2 mm (0.365 inch) and the bullets are not interchangeable with other 9mm rounds even if using the appropriate case.

There are also a bunch of wildcat and oddball 9mm cartridges out there such as the .38 ACP (not to be confused with the .380 ACP), 9x21 mm IMI, 9x25 mm Mauser, .38 Super, 9 mm Winchester Magnum, .356 TS&W, et cetera, that you probably won’t encounter unless you are into collectible guns or very specialized competitive shooting. The same is true for other calibers; there are multiple types of .22, .32, .45 and other pistol calibers, and the wide array of 5 mm, 7 mm/.270 inch/.280 inch, 7.62 mm/7.63 mm/.30 inch, 8 mm/.338 inch are bewildering. You’d think that a few different cartridges of each size would be sufficient to cover all needs but like automotive batteries there a huge array of different specifications.

The point is that here are reasons for each of these nomenclatures but they are pretty confusing to someone not familiar with shooting and ballistics but are generally well understood by experienced shooters. You should always and only shoot the cartridges that your gun is chambered for because even if another cartridge seems to fit it may be overpowered or failure to function the firearm action reliably, potentially leading to damage or injury of the shooter.


Also, Stoner direct impingement system used on nearly all AR-15 pattern rifles (except for the HK416) is the touch of the devil. It is cheap to make and service, and now that there are much cleaner shooting powders it doesn’t tend to foul the action after two or three magazines but it still isn’t as reliable as a good gas piston action.