OK, but I’m saying extra propellant isn’t required to propel other propellant, which is what I think Michael Emouse was saying.
Given that the propellant has about 1/2 the velocity of the bullet, where do you suppose its kinetic energy comes from?
Ah, now I get it. It was the comparison to rockets that was throwing me off - where most of the fuel is used to propel other, not yet used fuel.
Originally, a submachine gun was a relatively light weight, auto-loading firearm, that could be operated by an individual soldier, and fired pistol cartridges. A machine gun was considered a crewed, auto-loading weapon, that fired higher velocity cartridges. Caliber size doesn’t matter.
However, writers, shooters, and nimrods in general, have used both terms interchangeably.
Propellant leaves the barrel at over 4000 fps.
As a rule of thumb, handgun rounds have muzzle energy <1000 J. This is what an average person is comfortable shooting, without having the weapon braced against the shoulder.
There’s a number of large caliber revolvers (Such as the .454 Casull) that generate way more than that. And they don’t need bracing against the shoulder.
I’ll grant you, few people are comfortable shooting hand sized artillery. 
The solid propellant is converted to gas. Depending on the composition and shape of the propellant, the gas expands somewhere around 5000 fps.
Inside the barrel, the unburnt propellant would have the same velocity as the bullet, at least until the bullet exits the muzzle and the expanding gas pressure blows the gun shot residue past the bullet.
I think the original distinction (from the WW2 era) was that a submachine gun is small enough to carry around and fire while hand held, while a “full” machine gun was large enough that it had to be mounted on a tripod or vehicle.
Firing a machine gun that you are holding in your hand is something that is only possible in action movies.
And there are ‘pistols’ that fire .308 winchester and all sorts of other fun too. The question is usually whether anyone wishes to fire them without a brace, and if it is practical to hit anything other than the ground with them when used in that way.
The real fuzzy area is between compact carbines like the M4, AK74SU, short-barrel Tavor etc. firing small rifle cartridges versus the slightly beefier end of the sub-machine gun spectrum. It wouldn’t surprise me if an MP5/10 firing 10mm Auto out of a 225mm barrel is more powerful than an M4 shooting 5.56x45 out of a 370mm barrel.
But to answer the OP question is fairly straightforward and doorhinge has it right.
If it fires rifle ammo, is generally used from the prone position, and there are other people helping carry the ammo and relying on it for fire support, it is a machine gun.
If it fires handgun ammo, is fired while standing/moving, the ammo lives in the users pouches only and it is a purely individual weapon, then it is a submachine gun.
If it falls somewhere in between those definitions then something unusual is going on that the people involved may regret.
By this standard, I have seen a derringer firing 308 Winchester.
Almost certainly the much loved .22LR (Long Rifle).
Historically it’s not that uncommon to have both a pistol and a rifle, usually a lever action carbine, chambered in the same caliber of pistol ammunition. Pistols are easier to lug around all day, rifles get the most performance out of the cartridge.
Pistols chambered for rifle rounds exist too (the .454 Casull mentioned above), mostly just for the fun of spraining other people’s wrists.
CMC fnord!
:smack: You’re right. Primer is at the rear of the cartridge, so combustion proceeds from there toward the bullet.
In the first half of the 19th century and earlier, smoothbore muskets were the dominant firearm, since they could be loaded much more quickly than rifles. Since black powder is less powerful than modern gunpowder, their effective range was also much shorter. This combination was the reason behind armies advancing in close formation and firing at distances of much less than 100 yards. Muskets were much less accurate than rifles, but volume and rate of fire were the main concern.
Realtively few soldiers were killed outright in black powder battles; most deaths occured afterwards as a result of (sometimes minor) wounds and amputations.
And the common word in German for what we call a SMG translates to “machine pistol” (Maschinenpistole, as contrast to the regular MG being a Maschinengewehr). Thus the ubiquituous “MP-5”.
And in the interwar and WW2 period the different armies fielded the Thompson, M3 (“grease gun”), Sten, MP 38/40 and PPSh submachine guns quite widely.
FWIW, the old East German military type-designated their AK’s as “MPi-K”, I suppose for political reasons (not using the Nazi-era “Sturmgewehr” designation).
I beg to differ; submachine guns were not widely used in warfare until after the start of WWII, due largely to the conservatism of military bigwigs. In the German army, for example, the average grunt was issued a rifle largely due to Hitler’s insistence that it was good enough for him in the First World War. The British Sten gun was rushed into production as a stopgap measure, while Tommies continued to be issued bolt-action Enfields. In the US, the Thompson was considered the firearm of police and gangsters, while the BAR had been designed for the sole purpose of clearing trenches in WWI.
Pistols, BTW, have never been a serious weapon of war. They’re mostly carried by officers as symbols of rank (and were earlier used in duels). When the enemy is close enough for them to be used in self-defense, it’s time to get the hell out of there!
I believe .39 grams (6 grains) and 2.98 g (46 gr) powder, respectively.
To muddy the waters more, the term “personal defense weapon” is also used for bottleneck small-caliber SMG-types. Popularized by the FN P90 when Hollywood needs a scifi looking weapon, but most look more conventional.
While we’re at it, “Machine” Elf versus “Submachine” Elf?
Smooth bore flintlocks were the main infantry weapon from the 1600s up until the mid 19th century. Armies continued using smooth bore weapons even after rifles became common, since black powder quickly fouls the barrel and makes reloading difficult. Rifles were commonly used by hunters due to their increased accuracy, but a hunter could afford to clean his rifle after a few shots. A soldier couldn’t take a time-out on the battlefield to clean the powder fouling his barrel, so they stuck to smooth bores. Black powder makes a lot of smoke, so after the first volley of shots the battlefield was so thick with smoke that the better range of the rifle wasn’t of much use anyway.
Smooth bore muskets have a short range because they pretty much always fire curve balls. The round ball is going to randomly strike the side of the barrel as it travels down it, giving it a somewhat random spin. It will go straight for maybe 50 to 75 yards. After that, which way it goes is anyone’s guess. They used to say that you could stand a hundred yards away from a single musketeer and not fear getting shot by him.
Muskets were plenty powerful, though. Their range was not limited by power. These things were basically miniature cannons. I own a replica 1756 British infantry musket. It’s 46 inches long and has a .75 cal. barrel. While black powder does have less energy than modern gunpowder, you shove a rather large amount of it into the musket.
The Minie Ball solved the slow loading problem for rifles. Despite its name, a Minie Ball isn’t ball shaped. It’s a conical bullet with a hollow skirt at the back end. It is sized smaller than the barrel so that you can easily load it even after the barrel gets all crudded up with powder residue, and when you shoot the musket, the hollow skirt (being soft lead) expands and grips the barrel’s rifling. Minie style rifles were the standard firearm of the Civil War, and they used the same black powder that had been in use for ages. Not only was the use of black powder not a limitation on their range, but they were actually probably the worst standard infantry weapons to get shot with, ever. They produced wounds which were more severe than both the round balls that they replaced as well as the cartridge weapons that followed them. As for range, the Model 1853 Enfield that I own has an adjustable sight that goes out to 900 yards in 100 yard increments. Realistically, though, hitting anything beyond 600 yards is mostly a matter of luck (and with my old eyes anything beyond 100 yards makes me very happy). That ain’t too shabby for 19th century musket technology. The energy in the powder did not limit their range.
Getting back to the topic of this thread, several “machine guns” of sorts were invented during the Civil War. I put “machine gun” in quotes because these weren’t true machine guns which automatically cycle. They were mostly hand cranked weapons, though there were some other designs as well. A lot of people have heard of Gatling guns, but there was others like the Agar Gun (aka the Coffee Mill Gun because it looked kinda like a coffee mill). These weapons really impressed the heck out of Abraham Lincoln, but even though they were extremely impressive, they weren’t used much at all during the war. The reason was that ammunition was too difficult to produce and therefore too expensive. Since they couldn’t afford to just spray machine gun fire all over the battlefield, the few repeating guns that they had were left in defensive positions like bridges, mountain passes, and other natural choke points, where their huge expenditure of ammo wouldn’t be wasted. As such, they never got to prove their worth on the battlefield.
In the latter half of the 19th century, advances in metallurgy and manufacturing techniques allowed for the relatively inexpensive production of brass cartridges. Only after this did the modern machine gun become cheap enough to shoot that it was deemed a practical weapon, and once it was actually used on the battlefield it quickly proved its worth.
Machine guns were certainly valuable on the battlefield, but they had their drawbacks. They were big and heavy and their ammunition was big and heavy and they went through a lot of it. A few folks (all around the same time) got the general idea to use smaller pistol style ammo so that you could still get the high volume of fire of a machine gun (even if a bit less effective due to the smaller rounds being used) but also have something that could be much more easily carried, reducing the logistics requirements significantly.
As has already been pointed out in this thread, there’s a bit of variation in what is actually labeled a submachine gun. The term “submachine gun” was coined by General John T. Thompson, the inventor of the Tommy Gun. He envisioned it as being small and light enough that the regular infantry guy could carry it instead of a single shot rifle, and its rate of fire would allow soldiers to quickly clear out enemy trenches. The Tommy Gun and its ammo ended up costing way too much for Thompson’s vision to become a reality, but the basic idea (a weapon with a high rate of fire that is light enough to be carried by a single soldier) evolved into assault rifles which is what armies use today.
During the black powder era, they were used by cavalry. Carbines for long-range fire, pistols for short-range fire, sword or lance for close combat.
VERY close combat. They were also inaccurate, difficult to reload, and often misfired. That’s why I say they were not a serious weapon.