Always read your Owner’s Manual carefully, paying special attention to the ammunition specified by the manufacturer for that model of firearm. If you buy a used firearm that doesn’t come with a manual, go to the manufacturer’s website and odds are you can find it for download. If not, contact their customer support (typically a 1-800 number) to obtain one.
My SP1s are marked ‘.223’. Except for the odd box of Remington .223, I’ve always used surplus 5.56 and have had zero problems in my Colts or my builds. I can’t imagine that Colt – or any other manufacturer – would have separate lines for making the same barrel in .223 and 5.56.
ISTR 30 years ago that new semi-automatic rifles could not be marked ‘7.62 NATO’ or a derivative thereof, and they had to be marked ‘.308 WIN’. Maybe even in 1979, which is when my SP1s were made, ‘.223’ was used for civilian firearms and ‘5.56’ was used on the military receivers?
The great thing about standards is that there are so many to choose from. 
In a lot of ways, the name is often rather arbitrary. While the name of the bullet often indicates the caliber (diameter of the bullet, in inches), this is often tweaked a bit to differentiate similar diameter rounds.
In the U.S., musket balls were given a diameter (.75 caliber, for example). This carried on to Civil War era Minie balls, which despite the name are actually bullet shaped.
The next “standard” to be developed was the caliber followed by the year of adoption. For example, you had the .45-70 round which was .45 caliber, adopted for use in 1870. The round was actually named .45-70-405 with the 405 referring to the weight of the bullet (405 grains). And just to add to the confusion, it was called .45 Government in catalogs, so these days it is often called either .45-70 Government or .45-70 Springfield, the latter name coming from Springfield Armory, where the round was created.
Similarly, the .30-06 round (spoken as “thirty ought-six”) is a .30 caliber round that was adopted in 1906. This round shows how standards evolve over time. The .30-06 cartridge is very popular for hunting, and over the years has grown more and more powerful due to improvements in gun powder. The round itself remains the same size. The issue with the more powerful modern rounds is that it screws up the gas blowback in the original M1 Garand to the point where you can actually damage the rifle. There are two ways to fix this. First, you can tinker with the gas blowback on the rifle itself to reduce the gas pressure, or second, you can buy special .30-06 ammo that conforms to the original 1906 spec.
Another standard is (diameter of the round x length of the round) in mm. For example, 7.62 x 51mm. In case you are wondering why a lot of rounds have 7.62 mm diameters, 7.62 mm happens to be exactly 0.30 inches or .30 caliber. Some people prefer the diameter x length nomenclature since it actually tells you a bit about the size of the bullet and isn’t just the diameter.
Sometimes you can have a nearly identical round with two different names. For example, when they were developing a new round for the M-14 rifle, the round ended up being named the 7.62 x 51 mm, which ended up being called the 7.62 NATO round after its adoption by NATO as a standard across all NATO partners. Winchester wanted to release the round for commercial use as well, and released the round before development of the M-14 was finished. As development of the round continued, they ended up tweaking the round a bit, so Winchester’s commercial version is slightly different than the 7.62 NATO standard that they ended up with a couple of years later. Winchester decided to call the round the .308 Winchester. You may have noticed that 7.62 mm is exactly .30 caliber but the round is named .308. Those 0.008 inches are probably below the manufacturing tolerance of the round, but calling it .308 instead of .30 allows Winchester to easily distinguish it from other .30 caliber rounds.
That illustrates how arbitrary the names can be.
Some weapons are built to handle the difference in the two rounds and can fire either 7.62 NATO or .308. But since the rounds are not completely identical, if you put a .308 round in a weapon designed exclusively for 7.62 NATO, you can end up with problems like the case splitting.
Similarly, .223 Remington and 5.56 NATO are essentially the same cartridge, with .223 Remington being the commercial version and 5.56 NATO being the military version.
And again just to show how arbitrary the naming can be, .38 Special and .357 Magnum rounds both have exactly the same diameter. The only difference between the two is the case length. They are given different diameters in the name just to differentiate the cartridges.
And if you need another example of how arbitrary the naming can be, a .38-44 revolver is NOT firing a .38 caliber round adopted in any year ending in 44. It is in fact a .38 caliber pistol built on a .44 caliber frame.
There’s no real rhyme or reason to it. And while some rounds are commonly referred to by their caliber (.50 cal, for example), even here you have to be careful. If someone says .50 cal with no other clarifiers, they are probably talking about .50 BMG. But the .50 Beowolf round is also .50 caliber in diameter, but is a significantly smaller and less powerful round.
Suffice to say, there isn’t a reliable “rule” for the numerical naming convention of cartridges. I have discovered it’s best to simply memorize the numerical names of the most common cartridges, and to not be concerned about what the numbers actually “mean.”
And, more or less the 9mm. Oddly the .38 S&W (obsolescent) is different. .361
or 9.2mm.
Yep.
Sometimes I chuckle when some TV show states authoritatively that a bullet is a 9mm just by looking at it, or worse, by the hole. You can tell by the casing, of course, and in a lab a 9mm can be differed. The 9mm is slightly smaller, but you can load a 9mm bullet into a .38 casing… but not the other way around. A friend did reloading and got ahold of like 10000 9mm bullets (not cartridges) and used quite a few for .38/,357 revolvers for plinking.
However, do note that .38+p is fine in a .357 magnum.
I had heard that the idea of the .38+p is that the LAPD didn’t want a “magnum” pistol so issued large frame S&W .38 revolvers, which indeed, were +p rated, iirc. The idea was to have nigh .357 power but not use the dreaded “magnum” word.
True, since the .357 Magnum is the 38 Special +P of the pre- +P world. And it some senses, it’s better since the physical size of the round prevented it from being loaded in standard 38s. But I was wanting to get the general rule out, especially since the poster I was responding too was talking about a 38 super semi auto.
Oh, while I’m back in the thread, I’ll throw out another category that may confound someone not used to the details for firearms, and that is PCC (Pistol Caliber Carbines). Most traditionally found in lever action carbines, there are now quite a few semi-auto carbines (think shorter than rifle, bigger than pistol) in 9mm/40S&W/.45ACP.
Basically you’re shooting a traditionally ‘pistol’ caliber and powder charge out of a platform with a much longer (but often shorter than a ‘conventional’ rifle) barrel, with the concurrent better performance in range, accuracy and power.
And, just to make it better/worse, yes, some of them fire .357mag/38 special, 44mag/44spl (normally the lever actions) or are +P rated for the 9mm/40s&w etc.
And speaking about lever actions, wait until you have to talk about blunt/LeverRevolution/soft tip rounds for tubular magazines. Which has to due with rifle rounds that load out of an underbarrel tube in which the round are lined up with the tips potentially in direct contact with the primer of the next round.
In all honestly, @Crafter_Man put it best upthread-
Suffice to say, there isn’t a reliable “rule” for the numerical naming convention of cartridges. I have discovered it’s best to simply memorize the numerical names of the most common cartridges, and to not be concerned about what the numbers actually “mean.”
If you ever decided to purchase or use a firearm, learn the details for the weapons you’re buying and using to keep yourself and others safe (and take a damn course with live fire training!), but don’t sweat the details of all the options out there beyond shopping for the right tool for the right job.
There are some slight case dimensional differences between the .223 Rem and the 5.56 x45 mm NATO as well as the SAAMI spec pressure being 15% to 20% higher in the NATO spec (SS109, L110, SS111) rounds. So while you can shoot .223 Remington safely in a gun chambered for 5.56x45 mm NATO, you should not shoot the NATO spec ammunition in a gun chambered specifically for .223 Rem. It is also generally recommended to not reload 5.56x45mm NATO spec ammunition although many reloaders do so and are just careful about inspecting the brass. (A split case won’t blow up a good quality gun but it does exhaust a lot of burning powder and can result in a bullet being jammed in the barrel.) Confusingly, a lot of AR-15 pattern rifles designed for the civilian market are marked “.223 Remington” even though they are built to the NATO spec.
I’m now guessing the o.p. wishes he hadn’t asked this question because the history and nomenclature of small arms ammunition the lineage of British Royalty in how confusing it is all is.
Stranger
Where are there so many very minor variations on ammo?
If you were designing a gun isn’t it better to conform to existing ammo? And, if not, then have a really compelling case for a wholly new ammunition size?
Sometimes there is a particular reason for a certain narrow range of performance—say, for long range sniping or varmint hutting—but often it is just someone trying to come up with some theoretically optimized ballistics that aren’t actually an improvement in the real world. It is also the case that there are often essentially parallel developments; the 10mm Auto and the .41 Action Express were both essentially efforts to produce .41 Magnum ballistics in a Browning-style short recoil pistol (actually, in both cases initially in a CZ-75 platform, the Bren Ten and the Jericho 941 respectively); the 10mm Auto saw modest success in being adopted by the FBI but proved to be too powerful for many people to shoot in anything but a full-sized pistol, and the .41 AE essentially failed for lack of commercial or law enforcement .interest. The .40 S&W emerged as a “10mm Lite” load capable of being chambered in high capacity pistol platforms originally designed for the 9mmP but in many cases the pressure and slide velocities proved to harsh, and the improvement in expanding bullet technologies made the superior flight ballistics of the .40 S&W largely moot compared to the 9mmP.
Like I said, there are long and intricate histories of how these similar but different rounds came to be, and unless you are well versed it just seems as if there is no rhyme or reason for so many different cartridges and loadings.
Stranger
Well, leaving aside the “build a better mousetrap” issue that’s been going on for well over a century now as laid out by @Stranger_On_A_Train, there’s also a huge different in platforms. Tubular magazines, revolver cylinders, semi-auto magazines - these can all have a different on how ammunition is fed into a firearm, which requires different characteristics (rimmed vs rimless, the LeveRevolution for tube mags I mentioned earlier) in the cartidge.
Then there’s different barrels based on application, generally rifled vs smoothbore (shotguns), which again, have different needs for the ammunition.
And then, leaving out platform, there’s application. Different charges, different bullet weights, different bullet types, they all do different things better/worse.
And then, moving on, you have the historical range some of these calibers have been produced and supported over - firearms wear out very slowly without hard use, and people regularly use firearms older than they are, by generations in some case. Plus new platforms to use well known, reliable rounds.
A good way to look at it is considering a semi-complete multi-tool set: you have imperial and metric, every though several of those are close. You might also have a dozen different sized and headed screwdrivers to meet a specific needs. 3 or 4 different sets of pliers, and on and on and on. Different tools for different jobs and assumptions.
Now the ONE thing you see a lot of multitasking for related to firearms is CLEANING products. Because I can clean my 22LR, .223, 5.56 nato all with the same sets of brushes and heads (and extendable rods). My 9mm/38/357 all use a singular set as well. So there are a few advantages.
I wasn’t actually doing a conversion. Just showing some common rounds are metric and some are standard units.
Up until WWII it used to be rounds from Europe or Latin America had Metric designations and rounds from the US or Britain/Empire used Imperial.
After WWII and the formation of NATO it gets a bit more complicated, but generally anything with a military origin from then on (regardless of where it’s from) will be Metric and anything commercial from the US will be Imperial.
For example, the cartridge the US’s newly adopted XM5 Sig Spear military rifle fires is known as 6.8×51mm in its military version or .277 Fury when sold commercially (ie to hunters/target shooters/etc).
Some names are figments of a marketing department’s imagination, some have just come from common parlance, some from desire to differentiate from other cartridges on the market.
At one time, during the black powder era, there was a roughly proportionate relationship between the amount of powder you put into the gun and the performance you got out of it. Listing the (often nominal) powder load told customers something useful. When smokeless powders came out this proportionate relationship disappeared, and listing the powder load no longer told people anything useful. A few cartridges devised at the start of the smokeless era acquired such names, but the practice thereafter faded out.
When Olin-Mathieson, which owned Winchester-Western at the time, decided to steal a march on the competition by marketing a commercial equivalent of the T65E3 cartridge which US Ordnance was developing to be the 7.62mm NATO standard, the military specification was not quite yet finalised. They decided to call their version “.308 Winchester”, probably because they already had a .30 Winchester Center Fire cartridge and wanted to emphasise that this was new and different. The American market did not like the metric system, and did not want to understand it, so calling it 7.62mm was probably not thought to be a selling point. It did result in chamber specs which were, and remain, slightly different, and in some instances can cause interchangeability problems.
Here’s an instructive graphic that shows different common rounds side by side. As will be obvious, the bullet diameter is only part of the specifications- the case length and especially what’s called the headspacing determines whether that cartridge can be chambered by a particular gun.
@Lumpy’s illustration also brougth to mind another point you’ll see - but hear less about outside of reloading circumstances. While people talk about brass casings (and you’ll see plenty flying in Hollywood and IRL if you shoot), it’s not always brass.
Depending on cost, country of origin and other issues, your casing (containing the powder and primer, and topped with the actually bullet) can be brass, aluminum, or steel. And if you reload, you’ll be talking about brass, or if you’re bitching, you’ll be complaining about poor Russian steel cased ammo and corrosion from their powder… sometimes.
You do that too, and once a caliber is chosen by one of the big military industrial complexes as their standard, then gun makers (including government arsenals) around the world will design and pitch to buyers (private and governments) a crapload of different guns optimized for firing that, because you know that’s where the sales are going to be.
#1 in your picture is “caseless” but it sure looks like there is a case (albeit different from the other bullets).
What does “caseless” mean?
It means that the “case” is actually a solid lump of propellant that’s supposed to burn in its entirety, leaving nothing to eject. It’s a technology that’s been a couple of years in the future for the past few decades. The advantage is that they’re supposed to be much lighter than cased rounds and jam less often because they don’t have to eject a spent casing; in practice, they haven’t yet developed a propellant that burns so efficiently that no gunk is left behind, leading to a lot of fouling, and they’ve had problems with overheating, as the ejected casings usually convey a lot of heat out of the weapon. Plus, if for some reason a bullet burns in a magazine, all of the bullets will burn, which can be uncomfortable for users.