Metric confusion.

Status
Not open for further replies.
M

Murray B

Member
Joined
Oct 20, 2006
Messages
27
Location
Lauglin City, AB
I'm not metric and do not plan to be. To me a metricator is like a fornicator, only metric. Still, I do not see how to convert calibres.

My fancy calculator says that a .223" bore is 5.66-mm and a .308" bore is 7.82-mm. So why does everyone say they are 5.56 and 7.62 respectively?

Also I do not understand how a kilogram can be mass and weight at the same time when the gravitational constant is not 1.

It's about time the world switched to decimal inches.
 
The same way that a .38Special bullet actually has a diameter of .357in.

And an 8mm Mauser is actually 7.92mm.

And on and on and on...

A cartridge's designation and its actual size may or may not be related.
 
Kilogram is only mass...a newton is weight.

Cant help you with your actual problem though :)
 
.308" = 7.82mm which is the GROOVE diameter of a .30 caliber rifle.
.300" = 7.62mm which is the BORE diameter of a .30 caliber rifle. The opposing rifling grooves are .004" = .1mm deep.

Much (not all) caliber nomenclateure is based on BORE diameter. You have to be aware of the difference between the BORE diameter of a barrel - that is, the size of the hole they bored before they rifled the barrel - and the GROOVE diameter which is measured from the bottom of one rifling groove to the bottom of the opposite rifling groove. (There are some geometrical exercises to go through to get a characteristic number if there is an odd number of grooves.)
US convention and practice, is to make the BULLET diameter at or near GROOVE diameter.

All of which is why Lazzeroni caliber designations look like nothing else on the market. They are based on GROOVE diameter in millimeters, not BORE.

Then you have to get into the background of why a .38 Special shoots a .357 bullet (black powder history) and why a 7.92mm Mauser is known as 8mm (Because Americans can't handle but one significant figure?) Then there are the advertising departments. No other reason that .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .223 Remington, .224 Weatherby, and .225 Winchester ALL shoot .224" bullets. And a .22 Savage High Power shot .227" bullets.

The problem is not that there is no system of caliber designation, the problem is that there are several and you just have to rote learn which is which.

A kilogram is a unit of mass. Use of a kilogram "weight" is a convenience comparable to the pound mass. Nobody would know what you were talking about if you ordered a newton of hamburg.
 
Because you're working the equation backwards. Think internal bore diameter, not groove diameter. Divide your millimeters by the rough conversion factor of 25.4.

That puts a 5.56mm bore diameter at a relatively tight but not unheard of .218", leaving say about .006" of a .224" bullet's diameter to fill the grooves.

Also puts a 7.62mm bore right at ".30 caliber," .30. .008" of bullet diameter to fill the grooves, which can range from .3065" on a tight barrel of the NATO persuasion to .311" or even a shade bigger on a barrel of the Russian persuasion.

At least, I think that's whats up with the math. I might be wrong.
 
Whether a cartridge is designated by the barrel groove diameter, the bore diameter, or some other number having little or no relationship to either is strictly up to the developer/inventor of the cartridge. .222 Remington, .222 Magnum, .223 Remington, 5.56mm, .218 Bee, .225 Winchester, .22-250, .220 Swift, .22 Short, .22 Long Rifle, all (and probably many more) use a .224" diameter bullet.

So why is a particular caliber chosen? Well, when the Army was developing a new cartridge in the late 1880's it came out .30" (bore), or .308" bullet (groove) diameter; that became the U.S. rifle and machinegun caliber for the next 80 years. Some years later, someone, thinking of lengthy experiments and esoteric ballistics, asked the officer in charge why he picked .30 caliber. The man replied that "it seemed like a good round number."

Jim
 
Thanks fellows for the informative answers and please note that a newton is also a cookie and new ton is a completely different weight.
 
Almost got it covered

Caliber designations (round names) may or may not be the bore diameter, groove diameter, bullet diameter, or they may be something else entirely.

If the manufacturer thinks it is a good name, then there it goes. Generally round have either a name close to their actual measurement, or close to what they once were.

US .38 caliber revolvers fire a .357/.358 in dia bullet. But once they used an actual .38 cal. The same for the .44, which are actually .429. Close to .43, but does .43 Magnum have as much appeal?

Names are chosen 1) close to caliber, and 2) so they won't be confused with another cartridge. .22 caliber centerfires have been named from .218 to .224-something. And then there are others.

Some rounds name includes the year of US military adoption (.30-06), others include the original parent case (.22-250 Remington, a wildcat .22 developed from the .250 Savage case, and years later standardized by Remington).

Old blackpowder rounds once had the caliber, powder charge, and bullet weight as their name, along with the maker (.45-70-405 Govt), which was shortened to just caliber and powder charge (.45-70), and this trend was carried through a few of the first smokeless powder rounds (.30-30 Win, .30-40 Krag).

There are lots more quirks, and metric calibers follow a different set of rules, generally being caliber x case length, in mm (7x57mm Mauser), adding the letter R for a rimmed case (7.62x54R)

These are general, and many exceptions exist. A great reference on the subject is Cartridges of the World (Barnes). Reloading manuals also usually contain a short history of the different rounds.

Hope this helps.
 
I do not understand how a kilogram can be mass and weight
Click to expand...

Mass is weight. Are you thinking volume?

As for cartridge nomenclature, more often than not it is a loose representation of actual bore or bullet diameter. Sometimes a number is chosen becasue it sounds good, sometimes to differentiate one cartridge from another using the same bullet diameter. There is no standard. You must know the spec's for each individual cartridge.

and why a 7.92mm Mauser is known as 8mm (Because Americans can't handle but one significant figure?)
Click to expand...

Becasue the original J. Patrone (or I. Patrone) cartridge used .318" bullets, while the later S. Patrone (JS) used the now common .323" (8mm) bullet. A .323" groove diameter less the roughly standard .008" for lands gives you .315", or 8.001mm.
 
Status
Not open for further replies.
Back
Top