Question about caliber names and measurements

[cstarr3]

So, I realize that the stated diameter in a particular caliber is nominal. Usually a 7.62 caliber actually has ~7.82mm diameter. Most 7mm bullets are closer to a diameter of 7.2mm, and most 6.8mm bullets are about 7.0mm. The 5.56 NATO shoots a 5.7mm diameter bullet. Even the metric/NATO designation of the .50 BMG (actual diameter = .510") is 12.7x99, instead of showing the more accurate 12.95mm. I notice that many rifle bullets are given names that make them seem between about 1 and 3 tenths of a millimeter smaller than they really are. And this isn't a purely Western tendency; the Soviet 5.45x39mm is really a 5.6mm diameter, the Chinese 5.8x42mm is supposed to have a diameter of 6.0mm, and the old Japanese 6.5x50 has an actual diameter of ~6.7mm. Is there a reason for this nominal diminution of rifle bullets?

I had this crazy thought that such specifications might be indicative of the diameter of the bullet's lead core's diameter before the jacket went on. Any thoughts, anecdotes or facts?

And just for those who might know, why is this sometimes the opposite in pistol bullets (.44 cal = .430", .38/.380 = .355"-.357", .32 = .312")?

 

[rcmodel]

Calibers are generally named for the diameter of the hole drilled through the barrel.

But the bullet is bigger to fill the rifling grooves.

An example is the 30-06, or 7.62 NATO / .308 Win.

Bored hole diameter before rifling is .300"
.004" deep rifling * 2 = .308" bullet size.

rc

 

[Sunray]

Trying to figure out why a cartridge is called what it is can give you brain damage.
Like RC says, it's usually the bore diameter, not the bullet diameter. Except for most inch named commercial cartridges. IE. the .243 Win uses a .243" bullet, but 6mm converts to .236". The 6mm Rem, aka the .244 Rem, uses the same .243" bullet. That's marketing.
The Russian 5.45mm is mostly about their supply system. Russian stuff is nearly always using dimension designations to differentiate between ammo types. They don't want the illiterate conscripts who order stuff getting the wring thing from the illiterate conscript who ships it.
"....44 cal = .430"..." .44 Mag and Special use .429" jacketed bullets and .430" cast. Still a bore vs rifling thing. With a whole bunch of marketing thrown in.

 

[briansmithwins]

I had this crazy thought that such specifications might be indicative of the diameter of the bullet's lead core's diameter before the jacket went on. Any thoughts, anecdotes or facts?

If only it was so orderly.

Caliber and cartridge designations are a mishmash of historical accidents, ancient marketing, carryover from black powder cartridges, metric/standard conversions, and lots of other influences.

There are no standards and there is no reasoning behind the designations.

BSW

 

[urbaneruralite]

Sometimes caliber is measured at the lands, sometimes at the grooves.

 

[jmr40]

The person who developed the cartridge names it whatever he wants. There is no real system although some have been attempted over the years.

 

[justin22885]

it can all be very confusing, and the fact that there is never any one rhyme or reason for any of it makes it nearly impossible because its a question that can be asked individually for nearly every cartridge out there

 

[Comrade Mike]

As said before some are old black powder designations. Like the 45-70, 44-40, 30-30, etc.

45 caliber bullet over 70 gns black powder, 44 caliber bullet over 40 gns black powder... You get the idea

 

[Jim Watson]

It's not that there is no system, it is that there are several and you just have to learn which one you are looking at.

Why, for example, is the .256 Newton bigger than the .257 Roberts?
Because the Newton is .256" bore, .264" groove/bullet like a 6.5mm; and the Roberts really shoots a .257-.258" bullet.

Then you must consider the advertising department, which leads to my favorite counting exercise:
The .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Rem, .223 Rem, .224 Weatherby, and .225 Winchester ALL shoot .224" bullets.
But the .22 Remington Jet and the .22 Savage High Power don't.

Trick question, how big is the bullet in a .500-.450 Express? .458", the British put the parent cartridge first, the neckdown second.

 

[Outlaw75]

As said before some are old black powder designations. Like the 45-70, 44-40, 30-30, etc.

45 caliber bullet over 70 gns black powder, 44 caliber bullet over 40 gns black powder... You get the idea

Union Metallic Cartridge started that. The .44-40 was originally marketed by Winchester as the .44 Winchester Centerfire (WCF). When UMC started making it they changed the designation to .44-40 so as to not give Winchester free advertising.

 

[Jim Watson]

Again, .30-30 is not a black powder designation, it is a smokeless powder designation in the style of black powder numbers.

And a lot of black powder designations included the bullet weight: .45-70-405, .45-55-405, .45-70-500, .45-75-420, etc.

 

[cstarr3]

Thanks all. The idea of measuring based on the original boring (the lands), while manufacturing based on the groves makes a lot of sense. It would also explain why there is a pretty consistent discrepancy between the nominal diameter and the actual diameter.

I am well aware that there are marketing aspects, as well as need to differentiate... Cartridges named ".45," for example: .45 Long Colt, .45 HP, .45 GAP, .45 ACP, .45 Rimmed, .45 Schofield, .45 Super, .45 Win Mag, .45 Wildey... .45 Remington-Thompson. I guess it is a little easier with metric, though far from clear-cut: 9x18mm Ultra/Police, 9x18mm Makarov, 9x19mm Luger, 9x19mm Glisenti, 9x21mm Gyurza, 9x21mm IWI, 9x23mm Steyr, 9x23mm Largo, 9x23 Winchester, 9x25mm Mauser, 9x25mm Dillon, 9mm Action Express, 9mm Win Mag. I actually kind of prefer an off-measurement naming convention to the necked-parent naming convention (I would prefer the ".288 Remington" to the "7mm-08"). But, hey, to each his own. I can see why companies would like to have something that is easy to differentiate from other cartridges.

Again, thanks for the input.

 

[Jim Watson]

By the way, the common nomenclature for the Makarov cartridge, 9x18, is deceptive.
It is really a 9.2mm with .364" bullets instead of the .355" of the true 9mm family.

 

[dbmjr1]

Technically there is no such thing as .45 Long Colt. It's .45 Colt. The Long part was a throw back to when the Army was issued two different .45 cal ammunitions.

You need to add .460 Rowland to your .45 caliber list.

 

[briansmithwins]

Then there are cartridges that go by multiple names but are identical: 9mm Luger, 9mm Parabellum, 9mm NATO, and 9x19mm all being different names for the same thing.

BSW