Are gun/bullet calibres measured in inches or millemetres?

It often depends on the origin of the cartridge. The 9mm Parrabellum round was developed in Europe and therefore is commonly known by its metric unit. Such calibers as .45 acp, .45 colt, .32 a.c.p., .25 and .22 were developed in the United States, which of course doesn't have the metric system as a standard. To further confuse the issue, there are usually european equivalents of some cartridges (for example .45 is 11X23mm or something like that). As if this isn't bad enough a lot of different calibers aren't even accurate. For example .45 acp is actually .451 inches, and .357 magnum is actually .355 as is .38 special and 9mm. Additionally shotguns are measured in Gauges, which is the amount of lead balls the size of the circumference of the barrel it takes to equal a pound (so a 20 gauge shotgun means that 20 lead balls the size of its barrel will equal a pound). You'll get used to the oddities eventually:)

The basic answer is both.Some of the world uses metric system(mm)(mainly european countries) where as the US and some others prefer imperial(like .45 means .45 of an inch).

Also generally you can name a cartridge anything you want if you make it, as long as it is close enough to the bullet diametre so people know what you are selling.

For example the 45-70, 458 winchester magnum, 450 watts and 460 weatherby all use exactly the same width bullets, that being .458 of an inch.

Or you can call use both system if you want. For example the military versions of the 308 winchester and 223 remington are the 7.62mm and 5.56mm NATO.

It gets confusing but basically you use the calibre just as a rough guide, and then research further to see what the gun is actually firing and whether it is what you want.

Both. Those which were developed in the USA use inches. Those from Europe use the metric measurements. There will not be a standardization anytime in the near future because neither side is willing to change their designations. The exception might be NATO which uses both designations for the same round. For instance, the .308 round is called both .308 and 7.62mm. Other than that, I think the good old .45 Colt will always be called the .45 Colt and the 9mm will always be called the 9mm.

Yes, they are. And there's even less standardization than that, though it isn't quite as crazy as in times past. For instance, all modern 22 caliber centerfires use .224" bullets, whether they're named 222 Remington, 225 Winchester, or any of the others. On the other hand, the 22 Savage HiPower used a .228" bullet. Many of the classic British calibers from a century ago were of unique calibers that were considerably off from the nominal.

Caliber referrers to fraction of an inch in the form of a decimal, but firearms and ammunition is developed and manufactured around the world. Last time I checked some countries use standard some use metric. This is why you have both mm and cal.

I think once the world has a standardized unit of measure the firearms will follow suit.

Both. For example a .45 caliber bullet = .45 inches in diameter--- a 9 mm bullet measures 9 mm in diameter.

lot of answers right- but to liven up and confuse the issue- some countries used the land/bore diameter of the barrel to name the cartridge, other countries used groove diameter. .270 Winchester has a .277 bullet or 7mm. .284 diameter is 7.2 mm but usually called 7mm after bore of rifle. 6mmH&H is slightly larger .246/.247 than 'standard' .243/.244 bullet, reloaders report some loss af accuracy unless they use rare factory bullets. 6.5 mm mauser bullets .264 standard sloppy fit in Mannlichers- reloder need to get Hornady .268 bullets for best accuracy. .30 Russiun groove is about .310/.311 so .310 bullets work best- exception is some Finnish model 28 or m30 rifles that got .308 barrels when made- Finns had a 7.62x53 cartridge that was same case as 7.62x54 Russian but with the .308 bullet- cheaper than rebarreling 30-50,000 odd rifles, just make some good ammo and issue to police and coast guard regiments that had the rifle assigned to them. Black powder era cartridges had bullet for outside lube same diameter as the cartridge case- .22 rimfires same pattern now. A little later the bullet was inside case lubed and smaller diameter to fit inside case- old Colt .38 was about .376 diameter same as case that fit in old 'Navy' capand ball cylinder, later bullet shrunk to .358 to fit inside case- the 38 special reflects the case diameter size of grandparent Colt case. .38 S&W case is about .384 diameter with a .364 bullet- same basicly as 38-200 Webley/Enfield service cartridge revolvers from 30s to about 1980s. 9mm Markovs have about same .364 bullet- sometimes cartidge boxes marked 9.1mmx18 Markov..

You are obviously referring to the Iberian 'Happens' of the late 1960's. During this period HaPiNiS was indeed measured by the British industrial measure of the 'smile on our Beryls face'. The spread of British womanhood flying to Spain and other sexual cess pits meant that grubby little strumpets from Carlisle to just outside Carlisle were subject to this Latin experimentation. The inches you refer to are in fact the ancient Greek term 'Inches'. These were the measure between the eastern edge of an orifice and the western edge of belief. hence 'Our Beryl'.

They are both..... like you said... 9mm then you have .45 caliber.... why isnt there a standard system, because there is America and Europe..... America has standard english so we will use fractions and decimals..... Europe uses the metric system so they will always be millimeters.... 7.62 mm is a classic is their firearm rounds......

One caliber is one one hundredth of an inch so a .45 caliber is .45/100 OF AN INCH.