My little lad asked while playing 'Call of Duty' what is the difference between a machine gun and a sub-machine gun ??

What was your explanation to him? Submachine gun. A lightweight automatic gun that shoots pistol ammunition, is usually fired from the shoulder or hip, and often has the capacity for shooting single rounds. For example the M1 Thompson .45 cal. Submachine Gun. Machine Gun.A rapidly firing automatic gun (often mounted).For example the Browning M2 .50 cal. Machine Gun

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For the first time I have seen "History" at close quarters,and I know that its actual process is very different from what is presented to Posterity. - WWI General Max Hoffman.

My little lad said thanks! I said to him i thought a submachine gun was smaller with a slower rate of fire.

I've often wondered that myself. I thought a sub-machine gun was a smaller, portable machine-gun. Usable and carriable (yeah, not a real word) by one man.

A machine-gun was a huge, chunky, but immobile firearm capable of firing several rounds between reloadings (something like a belt-fed gatling gun or something).

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As stated sub machine guns although not necessarily light are easily caried by one man and initially fired pistol caliber rounds (often somewhat hotter than you would want to fire from a pistol however). Machine guns can be put into several different categories:

Light machine guns: rifle caliber guns that can be carried by one man and fired on the move although the preference is to use a bipod or tripod. May be belt or magazine fed. Examples would be the Bren historically or the SAW in modern times. Examples would be the Thompson, Sten, Uzi, etc. Smaller ones are sometimes called machine pistols.

Medium machine guns: Usually around 8mm rifle caliber rounds (in WWII and earlier often the same as used by service rifles and LMGs). Usually belt fed and operated from tripod. Not easily fired on the move unless vehicle mounted. Sometimes water cooled. Higher sustained rate of fire than LMGs.

Heavy Machine guns: Usually 12-20 mm although occasionally water cooled ~8mm guns are included in this. Fired from a mount or tripod. Examples include various 12.7mm guns, the Soviet 14.7, German 15mm, etc.

Not all ww2 machine guns were immobile such as the this.

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For the first time I have seen "History" at close quarters,and I know that its actual process is very different from what is presented to Posterity. - WWI General Max Hoffman.

lwd, close but not entirely.

A sub-machine gun is lighter then a LMG, and fires a smaller round (like a pistol round). Uzis, Thompsons, etc are examples.

A LMG (Light Machine Gun) is heavier, but still operated by one man, and can be belt or drum fed. It fires a larger round (like a modern day SAW).

Not sure if there is such a thing as a Medium Machine Gun.

A HMG (Heavy Machine Gun) can fire armor piercing rounds, is usually operated by a team, is mounted (on a vehicle usually) and is belt fed firing large rounds. a .50 Cal Machine Gun is a good example of this.

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I always thought that a Medium machine gun was a 30cal and a 50 cal was a Heavy machine gun.

__________________
They shall grow not old as we that are left grow old Age shall not weary them, nor the years condemn
At the going down of the sun and in the morning
We shall remember them. Lest We Forget

Actually, the terms "submachine gun" and :machine gun", though still in use, no longer serve as useful or descriptive classifications. Originally (circa WW I), a "submachine gun" was a shoulder fired, one man weapon, chambered for a pistol caliber round, and a "machine gun" was a crew-served, mounted weapon chambered for a rifle-caliber round.

But even before the end of WW I, the search for heavier firepower in highly mobile weapons began to blur the lines between submachine guns and machine guns. One of the first was the Browning BAR which was a shoulder fired, one man weapon chambered for a rifle caliber round, and capable of firing armor piercing bullets. Then came air-cooled, shoulder fired, bipod-mounted machine guns chambered for rifle-caliber rounds and
capable of being picked and fired on the move or used in static defensive positions. The development of "intermediate" (between rifle caliber and pistol caliber) cartridges further blurred the distinction since these combined the capabilities and advantages of the submachine gun and light machine gun without supplanting either.

The heavy machine gun (either air or water cooled) retained some important capabilities, such as sustained heavy fire and large caliber hitting power, which couldn't be built into either light machine guns or submachine guns, and thus remained in the arsenals of almost all armies. Today, machine guns and submachine guns are not strictly divided as they once were, and there is a whole spectrum of weapons from very small .32 ACP caliber submachine guns up to very large, vehicle mounted, multi-barrel, 30 MM machine guns, with no easily defined dividing line between them.

The M1919 and the M1917 both originally fired the 30.06 cartridge. The M1919 was classified as a light machine gun whereas the water-cooled M1917 was considered a heavy. The classification was due to the role the weapons were intended to play, with the M1917 placed in a static position, requiring sustained firing capability and the lighter, air cooled M1919 being more mobile and less able to fire a prodigous amount of ammo before the barrel got too overheated.

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Best Regards,
JW

Haha, playing too much ASL?

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Ok where was I wrong?
Historically yes but I believe that there are some modern smg that fire rifle caliber rounds.
Sometimes they are operated by one mand sometimes two. In particular if belt fed they often have a 2 man crew. In any case there is usually at least one additional man who's carrying ammo for the LMG.
30 caliber tripod or vehicle mounted mounted machine guns are sometimes referred to as such.
MMGs and even LMGs in WW2 fired AP rounds as well. Water cooled 30 caliber machine guns were/are sometimes referred to as HMGs. During WW2 especially the early stages HMGs were more likely to be tripod mounted than vehicle mounted.

One of the problems here is the terminology changes over both time and space. The defintions are also some what flexable. For instance should the M3 carbine be considered an SMG? But for it's heritage as a direct decendent of the M1 carbine which was not automatic it would seam to meet the critieria. What about the current US M4 carbine?

A submachinegun is also known as a machine pistol. The reason for this has to do with its size, orgins, and ammunition.
Submachineguns / machine pistols evolved out of automatic pistols. Both use pistol ammunition as opposed to rifle ammunition like machineguns that evolved from rifles.

Machineguns are somewhat hard to classify but generally one can use the following:

Light machinegun: Air cooled. It is portable by one man and uses basic "iron" sights. Its ammunition supply may be belt or box fed.

Medium machinegun: These are mounted on a rigid base of some sort and are crew served. The base gives the gun more stability increasing its accuracy and range. Many have more complicated sighting arrangments like telescopes or dial sights (for indirect fire) and they can be laid to fire along predetermined lines like an artillery piece. They also generally have the same caliber of ammunition as light machineguns do. Sometimes they are referred to as heavy machineguns.
They also are often water cooled instead of air cooled. This allows them to fire sustained bursts. They are almost always belt fed.

Heavy machinegun: These are usually larger caliber weapons like the M2 Browning .50 machinegun. The only real difference between a "medium" and a "heavy" machinegun is the caliber of the ammunition. Otherwise, both are identical; hence the reason mediums are sometimes conflated with heavy machineguns.

I think of submachine guns as pretty much requiring two hands to use while machine pistols are usable with one. But that's just me.

Some lmg's were clip fed.

These sort of definitions changed over time and space and from person to person. For instance is the BAR a lmg?

I think I've seen water cooled rifle caliber machine guns included in the heavy mg category. Where the air cooled version wasn't. May have been based on planned mission more than caliber.

ok,

reading this thread, I had expected this to come up:

Where does the German Sturmgewehr fit in?

You surely could not just classify it as a riffle (="gewehr")?
The Germans at first even called it a "Maschinenpistole 43" (="sub machinegun"). But it is also not 'just' an submachinegun.

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We have threads on that, but it is classed as a sturmgewehr(STG) or Assualt rifle, much in the same catagory as the AK47 or the M16.

__________________
They shall grow not old as we that are left grow old Age shall not weary them, nor the years condemn
At the going down of the sun and in the morning
We shall remember them. Lest We Forget