Hiram Maxim (someone who's history will not be done justice with one paragraph) was born on February 5th, 1840, and can effectively be credited with remapping the human landscape in a single lifetime. At 14 he began his life's path as an apprentice coach builder, and a few years later took up inventing, which seems like it was actually a viable career in the late 1800s.
Maxim, as I've said before, remapped humanity, or at least remapped the way humans interact with their world. He is credited with many mundane inventions such as the Curling Iron and the Asthma Inhaler, and a few more prominent ones like the Mouse Trap and the Carnival Amusement Ride. He attempted to lay claim to the Lightbulb, but got into a lengthy legal battle with Edison over it and lost interest. He toyed with powered flight and built a Steam Engine that actually flew a plane... a few feet.
But the one thing Maxim made that really threw a wrench in the gears of the world was the Machine Gun. And he didn't stop there.
To really see where this is going, we need to take a step back and learn how exactly a machine gun works, and to do that, we need to look at exactly what a machine gun is.
A machine gun is basically just a regular gun that has been modified to spew bullets at an alarming rate (as if one bullet isn't alarming enough). But what modification makes a gun shoot faster? Simply put, in a normal gun you have a barrel, trigger and firing pin. A series of springs and levers move the components so that when the user pulls the trigger the gun fires and a new round is lined up in front of the firing pin. Early machine guns like the Gatling Gun used a hand crank and multiple barrels and firing pins to do this single task in rapid succession. But these guns were not true Machine Guns. In a true Machine Gun, the user pulls the trigger once, and the gun continues to fire until the trigger is released, or there's no more ammo left.
And this is where Maxim's genius shines through. Every time a gun fires, the bullet goes forward and the gun goes backwards. In fact, as a child, Maxim was literally knocked over by the recoil of a rifle he was firing. That experience lay dormant for years, until inspiration struck. Maxim developed a way to use the action/reaction of a gun firing to "reload" the gun. In Maxim's gun, when the bullet fires and the weapon kicks back, it ejects the spent round, loads a new one, and reactivates the bolt, all in a single motion. What an elegant use of otherwise wasted energy.
There are 3 main ways of using the energy of the bullet to re-cock the gun in use today. Maxim gets credit for developing ALL of them. It's lengthy, technical stuff that's better delved into on a personal whim on how it all works but suffice it to say that very little change has been made to what this man came up with in 1885.
Regular machine guns are full sized rifles like the M-16 battle rifle or the AK-47 assault rifle. Those guns shoot very large rounds at a fairly rapid rate. The downside to pissing huge bullets like that is that the gun is almost uncontrollable at full fire. And a big, uncontrollable gun is terrible for close-quarters combat. Most manufacturers limit the rate of fire to 3 round bursts, which saves ammo and allows the user to maintain a margin of accuracy. But if you want to clear a room, you need something smaller, faster and more manageable.
Cue the Sub Machine Gun. This little guy doesn't fire a rifle round but rather spits a smaller pistol round like the 9mm. The smaller bullet has less stopping power, but also less kick. The gun remains more manageable under full fire and since everything is more compact, can actually fire a lot faster. Plus, a gun smaller than a rifle but bigger than a pistol is great for interior combat and things like drug raids.
Add to this the 100 or so years engineers have had to tweak these things and the gazillions of dollars governments and drug lords have put into that development and you come out with some crazy shit. Go watch The Fifth Element and you'll get a good idea of where Sub Machine Guns will be in the next 10 years.
