How is a bullet's caliber measured?

[Earnhardtjr3829]

As a gun collector I understand that bullet calibers move up accordingly to size. I have also gotten to know most of the major calibers and have been curious how the numbers for the caliber are decided. I'm pretty sure it is according to some type of measurement, but which measurement? For instance my .17hmr is in the same casing as a .22 yet the .17 is a longer round with more powder. That's just an example though, I am curious about all calibers. Can anyone further educate me on this? I feel stupid for not knowing but figure it's better to ask a dumb question then continue not knowing. Thanks for any help.

[OneInTheChamber]

Your .17 HMR is a neck downed .22 Mag. case. Necked down means the case narrows in the front, thus a small diameter bullet is driven by a larger casing. A .17 Cal is 17/100th of a inch. It's not the same as a .22

Bullets are generally measured by their actual diameter. However, many rounds aren't the exact number they are phrased as. A .38 is actually .355 (same as a 9mm).

Hope this helps.

[FirstFreedom]

Caliber (strictly diameter):

American standard - bullet groove diameter, using an inch convention.
European standard - bullet land diameter, using a metric convention.

IINM.

Cartridge:

Much more complicated...there have been extensive threads on cartridge naming if you can find them with the search feature.

But you've got many American rounds that are simply the caliber plus the designer, which may be Remington, Winchester, Marlin, Hornady, PPC, Casull, on and on.

You've got the many old rounds which used the convention of xx-yy or xx-yy-zzz, where xx is the caliber, yy is the powder charge in either black powder, or later in smokeless powder, and zzz is the bullet weight in grains. Such as .45-70 gov't or .45-70-450, or .30-30 winchester, or .32-20, .25-20, .38-55, .38-40, .44-40, etc. The dashes are not pronounced; for example ".45-70" is just "forty five, seventy", not "forty five dash seventy". The gov't on the end of that one designates it as an official government (U.S. military) round.

You've got European and military rounds with the convention of aa x bb mm, where aa and bb are both millimeter measurements, and aa is the bullet land diameter, and bb is the cartridge case length from back to front, such as 7x57mm mauser, 6.5x55mm swede, 7.92x57mm mauser, 7.62x51mm, or 5.56x45mm, 7.62x39mm russian, etc. The "x" in these is read "By", as in "seven by fifty seven millimeter mauser", etc.

Then you've got random ones which like the others, start with the bullet diameter in inches or mm, but then use a random designation which one must learn on a case-by-case basis (much like learning irregular verbs in English), such as .30-'06 springfield, which means .30 caliber (actually .308 caliber), adopted officially in 1906 in that particular loading, by the U.S. government as the standard small arm / rifle round, and springfield added on the end because the U.S. arsenal in Springfield (Mass?) was the place where the round was created/adopted. Quite a few other off-the wall designations as well. Many rounds have multiple names - most with multiple names have both an American and European version of the name. For example the .380 auto in the U.S. is .380 auto or .380 acp or 9mm Browning short. In Europe it is 9x17mm or 9mm Kurz.

[rem33]

38 is .357 same as a 357 magnum, 9MM is .355

[MTMilitiaman]

Just try to remember that especially the American designations aren't always all that accurate. The .38 Special and the .357 Magnum are the same diameter and shoot the same bullets. The .357 SIG, however, shoots .355 caliber (9mm) bullets. The .44 Rem Mag is actually .429 caliber. The .45 ACP is .45 caliber, but isn't the same .45 caliber as the .458 Winchester, since the .458 Winchester is .458 caliber and the .45 ACP is .451 caliber. Same with .50 calibers. The .50 Action Express pistol cartridge is .50 caliber while the .50 Browning Machine Gun is actually .510 caliber. You have cartridges named after people, like the .257 Roberts and the .35 Whelen. The .35 Whelen, BTW, takes .358 caliber bullets rather than the .355 caliber bullets of the 9mm. You have cartridges named after a gun magazine, like Mr. Simpson's Shooting Times Westerner and Shooting Times Alaskan cartridges, cartridges named after the class of weapon they were created for, like the 10mm Auto, and combinations of all of these, like the .32, .380, and .45 Automatic Colt Pistol. The .30-40 Krag was named after the inventor and designates a .30 caliber cartridge with 40 grains of powder. The .30-06 Springfield designates caliber and year adopted instead of powder.

Converting them can be difficult too:

6mm = .243 caliber
6.5mm = .264 caliber
7mm = .284 caliber
7.62mm = .308 caliber, unless it is Russian, in which case it is actually .311 caliber
8mm = .323 caliber
8.6mm = .338 caliber
9mm = .355 caliber
10mm = .40 caliber
11.46mm = .451 caliber
12.7mm = .50 caliber

However, if you do the conversions, you quickly notice inconsistancies:

The .257 caliber (.257 Roberts, .257 Weatherby, .25-06 Rem) is actually closer to being 6.5mm than .264 caliber. The .270 Win is .277 caliber, and this actually makes it closer to seven millimeters than the .284 caliber family, including the 7mm Rem Mag. And if you do the math for the .510 caliber .50 BMG, it comes out at 12.95mm, so its metric designation should more correctly be the 12.95x99 instead of 12.7x99.

There are 25.4mm in an inch so to convert inches to mm you multiply inches by 25.4.

Basically designations are just approximations. Confused yet?

[Pointer]

Caliber is measured in 100ths of an inch... thru the maximum diameter of the bullet.

Metric bullets are measured in millimeters...thru the maximum diameter...

For example the 9mm bullet is the same as the .38 Special which is actually not a true 38 hundredths...

The .357 magnum is also not a .38 caliber... but it is the same caliber as the 9mm.

Rifle bullets generally have much bigger cases and powder charges than the same caliber bullet/cartridge for a pistol or a revolver... Unless of course the rifle was designed to shoot pistol cartridges...

Now when you are finished with that puzzle... The are some strange designations such as .30-06 and .308 which shoots the same exact bullets from the same cartridge which has been made shorter and less powerful for the .308...

That actually makes the .308 a 30-06 Short...

And then there's the 250-3000 and the 22-250 (Nothing like the .22 Long Rifle) and the 30-30 and the .27-06...

EDIT This wasn't meant to be correct... it was meant to be sarcastic...

[Desertscout1]

For example the 9mm bullet is the same as the .38 Special which is actually not a true 38 hundredths...

The .357 magnum is also not a .38 caliber... but it is the same caliber as the 9mm.

Sorry but that is not remotely correct.
In the first place, the 9mm and the .38 are NOT the same diameter. The 9mm is .355" and the .38 is .357 as is the .357 Magnum.
The .38 got it's name when it actually was very nearly an exact .38 caliber. It began a "heel" type bullet which is the about the same diameter as it's case. like a .22 LR. It evolved in design over the years but the name didn't change.

[T. O'Heir]

"...the 9mm bullet is the same as the .38 Special..." No it isn't. A jacketed 9mm is .355", cast .356". The .38 Special and .357 Mag use a .357" bullet.
"...how the numbers for the caliber are decided..." Usually by the marketing types. Sometimes it's to avoid confusion between cartridges. For example, the .38 Special case is 1/10" shorter than the .357 Mag. Other than that they're identical.
'Calibre' is a measurement of the inner diameter of a tube. In a rifle barrel, the bore is the size of the hole without the rifling. In North America the calibre is measured from groove to groove. European cartridges are metric and describe the bullet diameter plus the length of the case in mm's. As in 7.62mm x 51mm. (It's daft too as the .308" bullet used doesn't convert to 7.62mm's. It converts to 7.82mm.) Some European cartridges are named for the bore diameter and have nothing to do with the actual diameter of the bullet. As in the .303 British. The bullet measures .311" to .314". That'd be the groove diameter.
As you can see, trying to figure out why a particular cartridge is called what it is can make you crazy.

[FirstFreedom]

Caliber is measured in 100ths of an inch... thru the maximum diameter of the bullet.

Yes, actual bullet diameter, which corresponds to barrel/bore groove diameter. And well, sometimes it's measured in 100ths of an inch; sometimes (more often) it's measured in 1,000ths of an inch.

Metric bullets are measured in millimeters...thru the maximum diameter...

No I do not believe so, if by "thru the maximum diameter", you mean the absolute max bullet actual diameter. Instead of measuring actual bullet or groove diameter,the metric designations measure the smaller LAND diameter of the bore. For example a .308 bullet measures 7.82mm, NOT 7.62mm, so using the metric/european convention, it's named in millimeters with the smaller one (7.62mm), not the actual bullet diameter (7.82). This holds true again and again and again, if you start running the numbers on metric named rounds. The difference is nearly always the same: The metric name is between 8/1,000ths" and 11/1,000ths" smaller than the actual bullet diameter. The 0.20 mm is right in this range of about 10/1,000ths (or 1/100th) of a difference, in the case of 7.62x51mm.

Then yes, you have the many anamolies of american-named cartridges which, for marketing reasons, the name is a larger diameter than the actual bullet:

-".44 mag" and ".44 spec" are .429
-".38 special is .357/.358
-".454 casull" is .451/.452
-".460 rowland" is .451/.452
-".460 s&w mag" is .451/.452

and on and on.

[Pointer]

Thank you for the well-meaning corrections.

I was unaware of some of the details ya'll have mentioned...

However, please note that I was spoofing the way cartridges are designated and the complete lack of consistancy in that effort...

And I was sarcastic about the feeble attempts to educate the thread starter on a subject that is very confusing to a "tyro".

Your more precise stats support that idea quite admirably.

[Earnhardtjr3829]

Thanks alot for the info, but can anyone tell me what the "r" is in the 7.62 x 54r?

[dave0520]

The "r" means "Rimmed" I believe.

.30-30, .45-70, .357 mag./.38 special (and all cartridges designed to be fired in revolvers) are also rimmed, but in American cartridges the "r" is usually omitted.

[Bud Helms]

I read the opening post and went straight to the end to post this.

... have been curious how the numbers for the caliber are decided. I'm pretty sure it is according to some type of measurement, but which measurement? ...

'Looks like you made an assumption there, eh?

[Desertscout1]

The "r" means "Rimmed" I believe.

Actually, in the case of the 7.62x54R, it stands for Russian but for the .30-30 and others, it does, infact, stand for rimmed.

[BobGnKY]

Then you've got the big bore calibers that were/are used on ships.

the caliber in those cases is the bore diameter - barrel length (in number of multiples of bore diameter). For example, a 5 inch 38 (a common WWII caliber for close in defense) has a bore diameter of 5 inches, and the barrel is 38 times that diameter, (or 190 inches). The big guns on the battleships (NJ, Missouri, etc) were, I believe, 16 inch 50's.

fwiw

[tINY]

A few corrections:

7x57 - in english is pronounced "two-seventy-five Rigby"
30-30 is pronounced "thirty WInchester center fire"


Carry on....



-tINY

[silicon wolverine]

"Actually, in the case of the 7.62x54R, it stands for Russian but for the .30-30 and others, it does, infact, stand for rimmed"

Sorry no. according to "assault weapons" Vol.3, at the time the 7.62x54R was developed there was also a 7.62x54 rimless round. THe russians decided to go with the rimmed round because of ease of manufacture of a gun that headspaced on the rim vs. headspacing on the shoulder. The "russian" designation came in the form of ammo importers mistaking the "R" for russian because all the weapons that fired it were combloc.

SW

[Pointer]

American standard - bullet groove diameter, using an inch convention.
European standard - bullet land diameter, using a metric convention.

You all are confusing me...

Is the mic diameter of the 9mm the same as the mic diameter of the .357 ?

Or is the .357 land diameter different from the 9mm groove diameter?

Or are they in fact, totally different bullet diameters?

Damn, this is making me dizzy!

Doesn't the bullet HAVE to fill the grooves when fired?
Regardless of how it is measured or designated.

How is a bullet's caliber measured?

With a micrometer?

[FirstFreedom]

Well, the actual bullet diameter is going to be the same (or *should be* the same), as the widest measurement of the rifle's bore in which the bullet is shot. The widest measurement is the grooves of the rifling. The lands are in fact a few thousands narrower, smaller than the bullet itself, so that the bullet actually has to squeeze and deform a little to cause solid engagement of the rifling upon the bullet. For whatever reason, it appears that Europeans, being the nuts they are, don't measure the actual bullet diameter when naming a cartridge. They subtract 8 to 11/1,000ths. I have heard, and it makes sense to me, that this measurement represents the diameter of measured distance between the lands of the rifling in the gun they're designed for. I could be mistaken however.

[Jim Watson]

The problem is not that there is not a system for caliber measurement and designation, it is that there are several and you have to recognize which is being used. Sorry, it is largely a rote memorization exercise.

Consider that .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Remington, .223 Remington, .224 Weatherby, and .225 Winchester ALL use .224" groove diameter barrels and .224" bullets. That is what happens when the advertising department gets in the game. Oh, yes, the .22 Savage High Power does NOT shoot .224" bullets, it is a .227" or maybe .228" depending on who you read. And things change over time, too. The original .22 Hornets were in rechambered .22 lr barrels, .222". The early factory Hornets were .223" with matching bullets. That is too much trouble for the makers to bother with for the few Hornets sold nowadays, they just get the usual .224" barrels. You can still buy bullets of both diameters.

The idea that bullets should be of groove diameter is largely a (modern) American convention. There is a European school of thought that says when you drive a bullet from the cartridge into the rifling, there is metal displaced by the lands. So they make the grooves a little deep so the displaced metal has someplace to go. Unfortunately, there is also a European theory that says a "tight" barrel is more accurate, and some Continental rifles are rather undersize. This led to some blown up Sakos in the early days of the .243 Winchester cartridge. They made some .241" barrels that were TOO tight for the pressures.

[Desertscout1]

Sorry no. according to "assault weapons" Vol.3, at the time the 7.62x54R was developed there was also a 7.62x54 rimless round.

You may be right. I've found references that indicate both ways but, so far, I haven't really seen anything concrete. I thought it was "rimmed" for many years until just last year when I read some stuff indicating that the R was for Russian.

[Pointer]

Europeans, being the nuts they are, don't measure the actual bullet diameter when naming a cartridge.

9mm and .357...how does each bullet actually MIC out...?

I don't have a 9mm bullet so I can't MIC one...

[Desertscout1]

I think it has been listed several times already on this thread but it's .355" for 9mm and .357" for .38/.357

[FirstFreedom]

Well, actually I'm not sure what I said holds true to the same extent with pistol rounds as with rifle rounds. Let's see here. A "9mm" bullet is .355/.356 - let's call it .356. That's 9.03mm. So it does NOT hold true for pistol rounds apparently, because not only is the metric designation not 8 to 11/1,000ths smaller than actual; it's actually a smidge BIGGER. But, on the other hand, I believe you will find the principle *does* apply to rifle rounds, such as:

7x57mm mauser (.284 is 7.22mm, not 7.00mm - difference of 0.22mm)
7.92x57mm mauser (.321 is 8.16mm, not 7.92mm - difference of 0.24mm)
6.5x55mm swede (.264 is 6.71mm, not 6.50mm - difference of 0.21mm)
9.3x62mm (.366 IS indeed 9.30mm - so this is the *exception* to the rule)
7.62x51mm (.308 is 7.83mm, not 7.62mm - difference of 0.21mm)
5.56x45mm (.224 is 5.70mm, not 5.56mm - difference of 0.14mm - this one is much less difference - the odd man out....hmmm)
7.62x39mm and 7.62x54mm (.311 is 7.91mm, not 7.62mm - difference of 0.29mm)

etc.

the difference runs roughly 0.22mm, give or take, which is 8.7/1,000ths of an inch.

formulas are:

mm to inches: mm measurement, times 0.3937 = inch measurement
inches to mm: inch measurement, times 25.4 = mm measurement

[Pointer]

I think it has been listed several times already on this thread but it's .355" for 9mm and .357" for .38/.357

Thank you

I did read the thread...

But it got a little confusing when we started talking about groove diameter and lands diameter...
That's why I asked for someone to MIC them...

FirstFreedom did...

Thank you

I have a .45ACP... times 25.4 = 11.43 + 22... I think I'll call it the the 12mm Whoop-a$$