.308 inches ≠ 7.62 mm


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Blakenzy
March 14, 2013, 09:43 AM
Sooo, I am finally into centerfire rifle cartridges in practice, no longer theory only and I discovered that 7.62x51 mm rounds are actually more like 7.8 mm in diameter. What nonsense is this and why is it called a 7.62 (accurate to the hundreth of a mm :rolleyes:) when it clearly is not?

For a minute there I actually though I had purchased the wrong rounds :p. Measured in inches and they are exactly .308 of an inch.

FWIW I suspect the answer lies in the land-groove disparity but I would like confirmation.

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USSR
March 14, 2013, 09:50 AM
The Europeans measure bore diameter; we measure by groove diameter.

Don

BruceB
March 14, 2013, 09:59 AM
All cartridge "names" are purely NOMINAL.

trying to place actual measurements in the "name" for each round is an exercise in futility, and is also pointless.

Those of us who reload DO have to know and understand the dimensions with which we deal, but for the average non-reloader, it's more important that he simply knows WHICH specific cartridge his firearm requires.

.300......Savage? Weatherby? Holland & Holland? Winchester? WSM? WSSM? All take a NOMINAL .308" bullet, but even then there may be slight variations.

There's no real rhyme nor reason for the naming of cartridges,; it's the accumulated baggage of over 100 years of invention, adaptation, marketing and imagination.

It's up to us to cope with it, and it's safe to say that NO amount of criticism is going to change anything at this late date.

Start reading, because you have a lot of catching-up to do.

primalmu
March 14, 2013, 10:13 AM
Yup. Like the .38 Special having a bullet diameter of .357. And a .303 using a .311 (or .312"?) bullet.

Blakenzy
March 14, 2013, 10:21 AM
I kind of hoped that NATO standards and the metric system would be an exemption to the mish-mash mess of cartridge naming.

ford8nr
March 14, 2013, 10:27 AM
Just be aware the 7.62 NATO and the .308 are NOT the same cartridges when it comes to reloading due to thicknesses of the brass. The same powder charge in both cases can give you different results. In rifles the head spacing spec is different due to the NATO cartridge is used in light machine guns. Rifle Shooter magazine had a good article in the March/April copy.

Blakenzy
March 14, 2013, 10:38 AM
Just be aware the 7.62 NATO and the .308 are NOT the same cartridges when it comes to reloading due to thicknesses of the brass.

Ah yes, I have seen that mentioned a few times, with special mention to IMI brass... which happen to be the cartridges I bought.

BTW can anyone offer any info on these? They have the headstamp "IMI 09" and the NATO cross. Nothing more.

mavracer
March 14, 2013, 10:42 AM
I kind of hoped that NATO standards and the metric system would be an exemption to the mish-mash mess of cartridge naming.
Not even close

A-FIXER
March 14, 2013, 11:00 AM
It is this
IMI

Israel Military Industries, Tel Aviv, Israel

1

USSR
March 14, 2013, 11:02 AM
BTW can anyone offer any info on these? They have the headstamp "IMI 09" and the NATO cross. Nothing more.

Yep, suggest you reduce any published load data using .308 commercial brass (especially Winchester) by 2.0 grains during load development.

Don

Blakenzy
March 14, 2013, 12:01 PM
Can I assume they are non-corrosive?

USSR
March 14, 2013, 12:39 PM
Yep.

Don

BigG
March 14, 2013, 12:59 PM
Calibers are named by marketeers. No connection between actual size and name in many cases.

BigG
March 14, 2013, 01:00 PM
Yes. NATO ammo is non corrosive.

Reloadron
March 14, 2013, 02:00 PM
Sooo, I am finally into centerfire rifle cartridges in practice, no longer theory only and I discovered that 7.62x51 mm rounds are actually more like 7.8 mm in diameter. What nonsense is this and why is it called a 7.62 (accurate to the hundreth of a mm :rolleyes:) when it clearly is not?

For a minute there I actually though I had purchased the wrong rounds :p. Measured in inches and they are exactly .308 of an inch.

FWIW I suspect the answer lies in the land-groove disparity but I would like confirmation.
Take a look at the below drawings. The upper image is a drawing of the cartridge dimensions and the lower is the chamber dimensions.

http://bearblain.com/images/LC%20Chamber%20Test%200.png

The Bore Diameter is .300" or 30 caliber (7.62mm) measured land to land while the Groove Diameter is .308" (7.84mm) measured from groove to groove.

The outside dimensions of the 7.62 NATO cartridge and the .308 Winchester cartridge are identical. The NATO brass is thicker resulting in less case volume. This thickness is especially true in the web at the base of the cartridge. So when loading NATO brass the rule is to start low and work the loads up.

Even NATO brass will vary, try weighing several flavors of NATO and commercial brass. How much less case volume is a matter of who made the brass.

Ron

Jim Watson
March 14, 2013, 02:03 PM
You will find caliber-cartridge names based on bore diameter or groove diameter. A distinction that is sometimes lost on the Internet Generation. There are a lot of people who think the bore diameter of their .30 caliber rifle is .308" when that is the groove diameter. They will tell you real quick the difference between clip and magazine, though.

It is not that there is no system for cartridge names, it is that there are several and you just have to learn them by rote.

Lazzeroni is the only gunmaker I know of that refers to their rifles by groove diameter in millimeters. Their 7.8mm Warbird has the same standard .300" bore, .308" groove, and .308" bullets as any .30 caliber or 7.62mm or .308, they just express it in a way they think will draw attention.

Then there are many cartridge names arising out of the advertising department.
Consider that the .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Remington, .223 Remington, .224 Weatherby, and .225 Winchester ALL have .224" barrel groove diameters and shoot .224" bullets.
Older .22 Hornets, .22 Remington Jet, and .22 Savage High Power do not.

Oh, yeah, a .256 Newton is bigger than a .257 Roberts. The Newton was a 6.5mm/.264 but they called it by the bore diameter while the Roberts is a .25 caliber going by groove diameter.

MachIVshooter
March 14, 2013, 02:45 PM
Yup. Like the .38 Special having a bullet diameter of .357. And a .303 using a .311 (or .312"?) bullet.

.303 British does have a .303" bore diameter land to land.

The .38 nomenclature is actually the result of the transition from heeled bullets (like .22 LR) of true .38" diameter to the smaller .357"-.363" bullets of cartridge case inside diameter

As always, though, know the correct bullet diameter for your cartridge/bore. Nomenclature is just an approximation.

MCgunner
March 14, 2013, 03:14 PM
In America, it's a .308. Only commies use the metric system. :D

BTW, hand me that half inch right there....not there? Well, gimme a 13mm then. Good 'nuf for gubment work. :D

MachIVshooter
March 14, 2013, 06:01 PM
BTW, hand me that half inch right there....not there? Well, gimme a 13mm then. Good 'nuf for gubment work.

Conversely, a 1/2" socket on an eroded, damaged or undersized 13mm nut/bolt is a good idea before air hammer/cutting torch removal. Same with 12mm on 1/2", 14mm for 9/16, 9/16 on 15mm, 17mm on 11/16, 13/16 on 21mm, etc.

And you know you're been wrenching too long when you can identify metric vs. SAE by thread pitch...

(Helpful tidbit: For bolt sizes M5, M6, M8, M10, and M12/M14, Domestic makers like 7, 8, 10, 13, 15 & 18mm heads, Asian cars typically use 8, 10, 12, 14, 17 and 19mm heads. European makers use all of them; they also use M5 and M7 shanks. Jerks.)

littlelefty
March 14, 2013, 10:53 PM
I know you're talking rifle stuff, but I still remember the wide eyed look of doubt, confusion, and amazement when a co-worker could not wrap his head around the fact that a .357 revolver can shoot .38 special cartridges.

I did finish with, "but not the other way around, for a whole different reason".

There was more to the conversation after that, but the opening volley was priceless.

303tom
March 14, 2013, 11:14 PM
1/2 inch = .50 = 12.7mm................

TexasPatriot.308
March 14, 2013, 11:16 PM
just play it safe and fire what your firearm says it is chambered for....any mishap is your fault.

madcratebuilder
March 15, 2013, 08:47 AM
The US military has embraced the metric system and I support it every inch of the way:scrutiny:

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