Right man for the job, right here!
What is the distinction or diffrence between a machine gun and a cannon on combat aircraft?
There are many differing theories at the moment, I'll go through every single one of them.
This is the standard for the United Nations: any round larger than 13mm in diameter can be considered a cannon. Anything smaller should be considered a machine-gun. In this instance, it is an issue of caliber.
Some others might argue that a cannon shell explodes on impact. While they are not necessarily wrong, there are confusing factors with this assessment. For example. During WW2 the Italians and United States used their 12.7mm guns (.50 caliber) in many ways. The Browning M2 was the .50 cal gun for the US, the Italians used the 12.7mm Breda Safat. Both of these weapons were used on aircraft. The Breda Safats generally were used in pairs while the Brownings, later in the war, found themselves being used in as little as 2 guns (generally plus 4 additional .30 cal guns) up to as many as 8 or 12 M2's packed together. Sometimes with more than 500 rounds per gun. But the distinguishing factor here is the fact that both countries manufactured and used API rounds for their 12.7mm/.50 cal guns. API stands for Armor-piercing/incendiary. In other words, the round is designed to puncture through into an aircraft's interior and explode. Yet these rounds are fired from a gun which is below 13mm, making it still technically a machine-gun by UN standards. The Mauser made, MG-151/15 is a 15mm weapon, a cannon by UN standards, but it fires no explosive rounds. There are many different types of rounds, every one has their use. While in the lower calibers you don't normally find as many, there is no reason why they don't exist. Standard bullet types are Ball (solid), Hollow point, etc... (anti-infantry) However, in the air, you're looking at Ball, Armor Piercing, Armor Piercing/Incendiary, Incendiary, High Explosive, and tracer.
One thing I should also do is take you through the most commonly used types of machine guns and cannon. NATO does this in millimeters, so I'll do that as well with a parenthesized caliber.
5.56mm (.22 caliber) (thx SuperCell for correcting me)
7.62mm (.30 caliber)
7.7mm (also .30 caliber)
7.92mm (.303 caliber)
12.7mm (.50 caliber)
20mm (generally kept in mm)
37mm (generally kept in mm)
40mm (generally kept in mm)
below we get to AAA sizes:
75mm (generally kept in mm)
80mm (generally kept in mm)
85mm (generally kept in mm)
88mm (generally kept in mm)
120mm (generally kept in mm)
As you can see, when we reach the UN standard for cannon, things generally stop being measured in caliber, or inches. The diameter of a .50 caliber bullet is .5 inches, that's where it comes from.
Now for some historical examples:
German produced models of Bf-109's (same as Me-109's) used in WW2 were fitted with many different types of weapons. In their early stages (BF-109E's) they were fitted with two 7.62mm guns (MG17) in the nose and two 20mm cannon, (MGFF type) one in each wing. As the aircraft evolved and pilots increased in skill and experience, the aircraft became simpler to fly, and simpler to aim with. The result were less guns, but more powerful ones. The Bf-109F series not only improved performance characteristics, but it deleted the wing cannons and instead mounted only one inside the propeller hub. The Bf-109F-2 model used the two MG17 7.62mm guns as well as a 15mm MG-151/15, produced by the Mauser company. This was referred to as a cannon, although it only fired ball, armor piercing, and tracer rounds. Later on, when their opponents became wiser and better armored, such as with the though as nails American P-47 fighter-bomber with eight .50 caliber machine guns, the 109's needed better hitting power. To do this Messerschmitt got rid of the 15mm MG-151/15 and installed a different cannon, the MG-151/20. The MG-151/20 fired larger shells with the added bonus of High Explosive rounds. The hitting power of the Bf-109 became much more substantial after this change. Later versions of the 109 replaced the 7.62mm guns with 12.7mm guns and 13mm cannon. The gun in the propeller hub, normally the MG-151/20 20mm gun, was later replaced by a much more sinister MK-108, a 30mm cannon with devastating explosive power. The first German combat jets, Me-262's, had four of these powerful MK-108 cannon. A more commonly used fighter, the Fw-190A models, had two 7.62mm guns, two MG-151/20's, and two MGFF's. That's four 20mm cannon plus the two Machine guns. You didn't want to be raked by that fire any day. Yet the Americans, British, and Russians were having success with their weapons, the browning .50 caliber M2 and the Hispano 20mm cannons. Some Allied fighters were fitted with offensive guns as large as 37mm and 40mm! (look up the Bell-P-39 Airacobra and Hawker Hurricane Mk.IId for more details on those)
There are even more differences to pick out what's a cannnon and what's a machine-gun. This has to do with muzzle velocity, or, the speed of the bullet as it leaves the barrel. Cannon generally tend to have a lower muzzle velocity than machine guns because of the larger projectile they are firing. The rounds are also a lot heavier, so gravity, especially up in the thin air of the sky, makes shooting large diameter cannon from aircraft a bit more like tossing grapefruits or watermelons at an enemy plane than a Machine gun, which is like pelting grapes and blueberries at the target, much faster and more accurate. However, a watermelon smashing against its target will do a lot more damage than a blueberry, won't it? There's also the question of the rate of fire of a weapon. It's much easier to spit out lots of blueberries at a time than it is to spit out a lot of watermelon. The point? Cannon tend to have much lower rates of fire, generally within the 250 rounds per minute range, while some machine guns can pull rates of fire as high as 1200 rounds per minute, as with the case of the MG42 though commonly the rate was at 600 rpm. This isn't entirely reliable, though, because many machine guns were designed to shoot through the propeller arc, and the rate of fire was drastically slowed by the interrupter gear installed to make sure the bullets didn't hit the prop.
Finally, we must consider carrying capacity for the weapon. What do I mean, I mean how many rounds are in the belt for the gun. Now, this normally won't mean much, as things change all the time, but let's take a look anyway. Cannon rounds require a lot to be packed into them, and therefore are generally larger. Their size makes packing them into a belt harder, and there are usually less rounds in a cannon's belt than their would be in a machine gun. Taking the example of the Bf-109 again. The 7.62mm guns in the 109 had 66 seconds of continuous firing time if fully loaded. The cannon? Some sources say there were less than 12 seconds of fire. This isn't always a good source, though. As many planes have short capacity for Machine gun rounds as well.
As you can see, it's a tough decision and not always clear, but if I had to pick one, I'd say that the distinction is the drawn line at 13mm. Anything smaller is a machine gun, anything larger is a cannon.
Have a look at some of these pictures:
In this one, below, the first 5 rows are of bullets and cannon. The very last row is filled with tank and artillery shells.
http://www.quarry.nildram.co.uk/Collection1.jpgThis is a picture of heavy Machine gun and cannon rounds:
http://www.quarry.nildram.co.uk/HMG1.jpgThis picture displays some of the more common cannon rounds of the German Luftwaffe. One of the reasons Germany lost the war is that they had no standard ammunition types. Each gun required a specific type of ammunition, thus confusing the factories.
http://www.quarry.nildram.co.uk/replica.jpgsome 20mm cannon cartridges, the blue tipped one I actually have a hollowed out version of in my room!
http://www.quarry.nildram.co.uk/20mm1.jpgand finally, these two pictures show the more commonly used aircraft ammunition of WW2:
http://www.quarry.nildram.co.uk/WW2aircart1.jpg
http://www.quarry.nildram.co.uk/WW2aircart2.jpgI hope I've answered your question.
EDIT: I take it you're a Murder, She Wrote fan too?
"So do all who live to see such times, but that is not for them to decide. All you have to decide is what to do with the time that is given to you."
-Gandalf
[This message has been edited by Boosh (edited 05-20-2006 @ 11:01 PM).]