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Ok guys, slightly (?!) off-topic, and it will probably make me look stupid, but:

What makes a cannon a cannon?

7,7 and 14,someting mm are both machine guns, however, from 20mm onwards it's called a cannon (e.g. on a Zero).

I am wondering: is it just the calibre that matters (e.g. from 20mm and up it's a cannon) or does it have a different way of operating compared to (machine)guns?

Anyone know?

20mm was about the smallest projectile that could hold an explosive warhead. No difference in operation that I know of . . . other than larger the weapon the more the recoil we have to worry about.

And I'm guessing that at this point, with technology that makes some .50 cal projectiles explosive, the difference in nomenclature is mostly historical.

Interesting question. Wern't many guns througout history large caliber guns that fired only solid shot called cannons? What about the early medieval hand cannons? Apparently sometime in the 20th century the naming conventions were changed?

I thought anything above a .50 cal was considered a cannon.

That would be a reasonable assumption and I would also tend to agree. If you expand the original subject it gets more confusing. You have cannons, guns and howitzers. The 30mm mounted on A-10's. Is it a 30mm cannon or 30mm gun? I would guess it's officially a cannon, but most of the time it's called a gun. What about the 75mm mounted in b-25's most of the time I hear them referred to as guns.

I'm sure somewhere in the Dept. Of Standards and Measurements there is a set of documents that define the rules and naming conventions.

I think the basic distinction in WW II was could it fire an explosive round.  Though the Germans had a 15mm "cannon" in some of their A/C..., and I'm not sure that it did.   Or that it didn't.  

The Japanese and Soviets had 14.5mm machine guns, so the over .50 caliber is a cannon theory is out the window. If the Germans had the 15mm cannon, but didn't use exploding shells then the exploding shell theory is out the window.

Looks like the naming was arbitary.

This all reminds me of the old tootsie pop comercial with the owl and the question " how many licks does it take to get to the chewy center?" Some say 100, some say 500 and some say 1....

Doesn't this stem from the army difference between a gun and a weapon.  That one isn't that hard to find exactly what it is. 

I do remember a Gun is what you would call arty, tank main armament, etc.  What you carry is a weapon....

A machine gun fires a bullet, a cannon a shell. The main difference , aside from the general one of size, is rate of fire. Until autocannon, a cannon was a single shot weapon. A auto cannon uses either a machine gun mechanism or a gatling gun type.  Also another major difference is amount of ammunition.
 
In early WW2 the USN used a 1.1cal.  machine gun. I haven't hearn of any larger US gun.

Howitzer, gun and mortar are all the same thing (ie they fire an explosive projectile) but they differ in trajectory, velocity, barrel length and distance. As for the sizes, most ammo guys I know (myself included) consider 20mm and larger as projectiles because they can carry explosive or incendiary payloads, while .50 cal (14.5mm) and smaller are considered small arms. Yes, there are exceptions, but I think there are no set definitions because there hasn't really been a need for any. Any treaty or policy document I worked with usually spelled out the definition explicitly so everyone was on the same page, but I've seen both definitions on different documents too. Its the kind of fuzziness that gives lawyers an excuse to charge hundreds of hours. [8|]

My .02 anyway.

I think its down to pure nomenclature. The line has to be drawn somewhere.

Up to .50 its a machinegun.
Up to 14 inch its a cannon.
Over 14 inch its a CANNON.

From the internet wikki:

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellants to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees, depending on their intended use on the battlefield. The word cannon is derived from several languages, in which the original definition can usually be translated as tube, cane, or reed. In modern times, cannon has fallen out of common usage, usually replaced by "guns" or "artillery", if not a more specific term, such as "mortar" or "howitzer".

The Browning .50 was originally called 'light cannon'

Let's see if I can hijack this thread:
What is the better aircraft armament. A large number of heavy caliber machine guns or smaller machine guns and cannons? Why did everybody else (Brit's , Japs, Germans and Russians) use the small machine gun cannon setup and the American's use the 4,6,8 x 50 cal setup?

ORIGINAL: JohnDillworth

Let's see if I can hijack this thread:
What is the better aircraft armament. A large number of heavy caliber machine guns or smaller machine guns and cannons? Why did everybody else (Brit's , Japs, Germans and Russians) use the small machine gun cannon setup and the American's use the 4,6,8 x 50 cal setup?

LOL yes these are epic discussions.
Must have already covered a couple of pages in every historical wargame/flightsim forum.

The best is when everybody starts to draw the different ROF/weight/muzzle velocity/... graphs.

Interesting. Like watching some good gunslinger movie... [8D]

Here's what Wiki says on the subject, and I mention wiki because its usually only half right half the time:

When referring to cannon, the term gun is often used incorrectly. In military usage, a gun is a cannon with a high muzzle velocity and comparatively flat trajectory,[4] as opposed to other types of artillery, such as howitzers or mortars, which have lower muzzle velocities, and usually fire indirectly.[97][98]

The Germans at least did have a 13mm machine gun on some of their planes; that's roughly equivalent to the .50 caliber MG the US used.  IIRC the Germans went with cannon because they were more often attacking bombers, and it takes a lot of MG bullets to bring down a multi-engine plane.  The RAF did the same thing since their planes were attacking German bombers, although there were some proponents of the "hail of lead" that lots of lightweight MG's could produce.  A pilot didn't need to aim accurately to hit a target when you're firing 12 .303 MG's, after all.  Note that they quickly went to cannon and ditched the light MG's, though.  Ground attack is more effective with cannon as well, compared to light MG's.

So by this wiki definition the 30mm cannon on the A-10 is a gun because it fires a high velocity shell at a flat trajectory.

I've seen somewhere else in wiki that anything over 16mm is a cannon.

Ok so cannons are better for bringing down bombers and strafing. However is your strafing a massive troop concentration in the open is it better to have a slower firing cannon with a limited amount of shells or more machine guns with a higher rate of fire and significantly more bullet supply.

Come on guys. Whos the first to point out that the SU aces had extreme succsess in A2A with the 37mm? [:D]

ORIGINAL: sfbaytf

So by this wiki definition the 30mm cannon on the A-10 is a gun because it fires a high velocity shell at a flat trajectory.

I've seen somewhere else in wiki that anything over 16cal is a cannon.

Ok so cannons are better for bringing down bombers and strafing. However is your strafing a massive troop concentration in the open is it better to have a slower firing cannon with a limited amount of shells or more machine guns with a higher rate of fire and significantly more bullet supply.

Incorrect, Military usage a gun is a cannon with a high muzzle velocity and flat trajectory, so the A10's 30mm would be an Auto Cannon by definition.

Although Wiki does states there are a dozen Genre's of "caliber" weapons, its possible that up to 14.8mm is a "rifle bullet" 14.9 and up is a "cannon" and such.

Like for example a 75mm howitzer is a "Howitzer cannon" where 88mm flak is probably something else.

If only wiki gave the different caliber's of genre it would clear up what each branch of weapons are.