what is the difference between caliber and mm when speaking about gun bores?

Answer 1

Many of the finest cartridge designs originated in Germany or Austria. Naturally, they're given metric designations. The classic British cartridges and American ones tend to use English measurements. There are exceptions. The 30R Blaser is one German cartridge that uses English measurements, and the 7mm Mauser was so well established that the 275 Rigby designation for the cartridge never really caught on, and American cartridges that shoot the same .284" bullets are often but not always named 7mm something-or-other. Also, American military cartridges will have an English measurement but also, to comply with NATO and CIP rules will also have a metric designator.
But it gets more complicated than that, and you've already had some erroneous answers.
For one, there's no consistency about whether a cartridge uses bore size measured between the lands or the slightly larger groove size, and many use a purposefully inaccurate name to avoid confusion with other cartridges that shoot exactly the same bullet. Almost all 22 caliber rifles shoot a .224" bullet, whether they're named 218 Bee or 225 Winchester. Only a few old ones like the 22 Savage use .228" bullets.
A 9mm Parabellum uses a 9.02mm or .355" bullet, and that will be about groove size. A 38 caliber handgun will shoot a .357-.358" or 9.09mm bullet, not a .38" or 9.65mm bullet, for historical reasons. In the earliest days of metallic cartridges, there was one with the same case diameter as the modern 38 Special, but the bullet was bevel-based. That is, it had something of a mushroom shape so the rear part that stuck inside the case was just under 36 caliber, while the exposed part was 38 caliber. It wasn't terribly long-lasting in popularity, but you could do a cylinder conversion to take the case, use a normally-shaped bullet inside the case, and leave your 36 caliber barrel alone and come up with something that worked and saved a couple of bucks compared to buying a new gun. So it hasn't been a 38 caliber in ages, but the designator has stuck despite the historic reduction in the actual caliber.

Answer 2

Caliber is measured in so-many parts of an inch. For example, a 357 Magnum is .357 (three hundred fifty seven thousandths of an inch, bore diameter). Millimeters are metric. So a 9mm is a 9mm bore diameter. However, it's an old and complicated process, and sometimes it's totally illogical. For instance, a 38 caliber, is actually .357 bore diameter. That's why you can shoot 38's in a 357. They made the 357 round longer so it couldn't fit into traditional 38's. It would blow them up because it's a much more powerful round. The 38's, being less powerful, won't harm a 357. And the 38 got its designation in the first place because of the changeover to smokeless powder from the traditional black powder. In black powder days, the most popular revolver was the 36 caliber, which is what .357 is when rounded to the nearest hundredth. But when the world changed from black powder to smokeless, a new designation had to be found for the new, more powerful guns, so they bumped the caliber up to 38 (in theory at least), although in reality, nothing changed but the name. The bore diameter remained .357. I know it's confusing, but that's the way things are sometimes. Anyway, the basic rule of thumb is that they both measure bore diameter, caliber in inches (or fractions thereof) and mm in metrics.

Answer 3

Caliber is a measurement of the bullet diameter in inches. Well fractional inches anyway. MM is the metric equivalent of that. A 9mm bullet is 9mm wide and a .50 caliber is half and inch wide. You dig? If you want to compare the two, look up a decimal to inch conversion table on a machinist site or hardware store.

Answer 4

Folks are right about the inches versus millimeter explanations. However, someone stated that 9 mm bullets were .375" in diameter. WRONG!!!!!! Nine milimeter bullets, for the 9x19mm Parabellum and the 9 mm Kurz/Corto/Browning Short (aka .380 ACP or 9x17 mm) are .355 inches in diameter. Oddly enough, bullets fo the 9 mm Makarov are .363 inches in diameter.

Another person spoke of the oddity of shotgun gauges. Here is how the gauge system came about.

The practice dates back to the 15th or 16th Century and refers to the number of lead balls that could be cast for a given bore from one pound of lead. For example, a .775 inch bore would use balls cast at a size giveing 10 balls per pound, and a .723" bore used ball cast at 12 per pound, hence the 10 gauge and 12 gauge designation. Now days, gauge designation is restricted to shotguns, but as late as the mid 1800's, British gunmakers marked rifles, and handguns with gauge designations.

Why the .410 gauge shotgun? Me, I don't know! It is as useful as screen doors on a submarine, I don't know why it survives. Please note that the .410 gauge is written with a decimal point, unlike 10, 12, 16, 20, or 28 gaugees, since the .410 is the nominal bore diameter rather than actual gauge.

Doc

Answer 5

mm are a metric unit of measurement when you see .380 that is the measurement in english. Take for example 9mm the diameter is 9 millimeter which is roughly equivalent to .380 (american 9 mm as some call it) But the case length is different (9x19mm parrabellum) 380ACP is (9x17mm) DO NOT TRY TO FIRE ON IN THE OTHER!!!!!! Or you will get hurt!

Answer 6

Caliber is the diameter of the bullet which is usually in metrics so a 9mm means the caliber of the bullet is 9 millimeters.

Answer 7

Caliber is "SAE" and mm is metric.

A 30 caliber bullet is .30 inch dia.

A 38 caliber is .38 inch.

A 9 mm is 9mm dia.

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Then there's the shotgun "gages" which are really interesting... the number gets smaller as the bore gets larger....
except the "410" which is back to bore size... .41 inch.

I forgot how they came up with that...
something about how many loads you can get out of how much powder I think.....
when they were muzzle-loaders.

Answer 8

Definition of Calibre: The diameter of a rifle bore, measured in thousandths of an inch or in millimetres

So to answer the question the metric (mm) size of a rifle bore is a calibre

Answer 9

There is no difference. Traditionally Americans express caliber in inches or fractions of an inch. Overseas caliber is traditionally expressed in millimeters. Example: The .30-30 Winchester is called .30-30 Stateside; in Europe it is known as the 7.62x51(R) with 'R' meaning 'rimmed,' to differentiate it from the 7.62x51 NATO (which is rimless).

H

Answer 10

caliber is american standard .001". MM is metric. So a 9mm is roughly .375" which = 3/8" or about the same as a .38 calibur.