Why Are Bullet Dimensions Not Round Numbers?

[VetLegion]

Say, 5.56 mm or 7.62 mm. Why isn't it 5.5 or 5 mm? Or 7.6 mm?

Often when you encounter weird dimensions in the metric system
it is the result of some English or US measurement which became
the international standard. It sounds sensible in inches and a little
bit odd in centimeters.

However, trying to trace back bullet dimensions to their imperial
origins I discovered that those too are weird. 7.62 mm corresponds
to caliber .308, but why is that one not .300?

You could say that during the trials they discovered that a bullet
with diameter of some size has the best properties, but given how
far back all this goes I am skeptical about that. Could it be because
machining tools or lathes available 300 years ago had some peculiar
limitations?

This keeps me awake at nights, so I'd greatly appreciate light being
shed on the topic.

[onodera]

These are so-called lines ( http://en.wikipedia.org/wiki/Line_(length) ), tenths of an inch. So 7.62 is supposed to be 0.3, but different cartridges of similar calibre have often slightly different diameters. If you develop a new cartridge with a new length and internal structure, why care about the perfectly equal diameter?

[KrazyHorse]

7.62 mm corresponds to caliber .308

No.

[Felch]

7.62 mm by 51 mm is a .308. .308 Winchester is roughly the same as the 7.62 NATO. So he's partially right.

[Elok]

I'd always assumed that it was the gun barrels that were bored out (or whatever) to an exact width, while the bullets were slightly narrower so they'd fit in. Maybe a 9mm bullet is meant to fit in a gun with a muzzle exactly one centimeter wide, while a .45 barrel is pretty much a perfect half-inch. Or maybe not; I never really gave it much thought.

[KrazyHorse]

7.62 mm by 51 mm is a .308. .308 Winchester is roughly the same as the 7.62 NATO. So he's partially right.

Then somebody needs to work on their math. 7.62/25.4 = 0.300

[Wiglaf]

Depends on the calculator you use. I am using a solar powered calculator and get 0.306, but Google calculator chops it off at 0.3. I am never using Google Calculator again. You get what you pay for.

[snoopy369]

25.4 * 0.3 = 7.5 + .12 = 7.62

Looks right to me. 0.300

[Japher]

Blood comes out better in increments.

[Straybow]

7.62 mm = 0.30 in, 7.65 mm = 0.31 in, yet the 7.62x57 and 7.65x53 slugs can actually be used almost interchangeably.

.30 carbine, 30-06, .300, .301, .303, .307 and .308 are all much the same in diameter, they are given different names by convention. Some were originally designated by the size of the bullet itself (I think this is the case for the .308). Some were originally designated by the intended barrel design across the lands, which is necessarily smaller than the bullet size. That may be the case for the .303. The 30-06 was named for the 1906 Winchester rifle for which it was designed.

When bullets are made of different materials by the same manufacturer (simple lead ball vs copper jacketed) the bullet size is slightly different to get the same ballistic performance despite the difference in the way the metal will register in the rifling. Each manufacturer's bullets may be of slightly different size, and match shooters will swear this round works better with this rifle make but not with another rifle make, etc.

[Dr Strangelove]

Essentially the .30 caliber rifle bullet came first, so the 7.62 mm was made for the sake of interchangeability. Likewise the M-16 used a .23 caliber bullet so countires using the metric system developed the 5.56 mm bullet.

[BlackCat]

Wasn't the garand rifle designed with 7.62 as standard ? I might be wrong, but I think that that rifle was the prime course of using 7.62 as standard.

[Skanky Burns]

30-06 was the standard when the garand was designed (between mid 20's to mid 30's). 7.62 NATO was developed in 1950's and adopted by the US for use with the M14.

[Dry]

Say, 5.56 mm or 7.62 mm. Why isn't it 5.5 or 5 mm? Or 7.6 mm?

Often when you encounter weird dimensions in the metric system
it is the result of some English or US measurement which became
the international standard. It sounds sensible in inches and a little
bit odd in centimeters.

However, trying to trace back bullet dimensions to their imperial
origins I discovered that those too are weird. 7.62 mm corresponds
to caliber .308, but why is that one not .300?

You could say that during the trials they discovered that a bullet
with diameter of some size has the best properties, but given how
far back all this goes I am skeptical about that. Could it be because
machining tools or lathes available 300 years ago had some peculiar
limitations?

This keeps me awake at nights, so I'd greatly appreciate light being
shed on the topic.

I would say the round number might be the weight of the bullet, not the diameter.
Without being an expert, I'd say that the weight of a projectile determines the amount of powder you'll need.

So I guess, 300 years ago, people made bullets of a given weight.
The caliber was only a consequence.

What is the diameter of a sphere of X gram/ounce/grain of lead?

[Lul Thyme]

What is the diameter of a sphere of X gram/ounce/grain of lead?

X gram of lead has a volume of (0.088)X cubic centimeters.

A sphere with this volume has diameter (0.552)X^(1/3) centimeters (X^(1/3) is the cubic root of X).