A Machine Gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute so long as the trigger is held down. It is not a select-fire weapon; machine guns are usually full auto only.
Meanwhile, a submachine gun usually abbreviated to SMG, is a magazine-fed, automatic carbine designed to shoot handgun cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun.
In Mortal Kombat
Machine guns are used by police and military forces in Earthrealm. Jax Briggs has been known to use a Machine Gun in many of his appearances throughout the franchise. Officer Kurtis Stryker and Jax also had the weapon in Mortal Kombat: Armageddon.
Deathstroke is seen using a machine gun in Mortal Kombat vs. DC Universe. Jax also still makes use of one in the same game, with his 2nd Fatality and his victory animation having him use it. With the latter even having Jax shoot the screen.
In the storyline of Mortal Kombat (2011), Kano and his Black Dragon thugs stole a large number of high-tech & modern weapons including shotguns, machine guns and rocket launchers, then sold them to Shao Kahn, Shang Tsung and the Tarkata, aiding them in the Konqueror's new war on Earthrealm. In Chapter 12, Kabal and Stryker carry around MP5KA4s which they use during said invasion.
Originally in the MK (2011) Beta, Jax had an Submachine gun on his back but it was removed later on for unknown reasons.
In Mortal Kombat X, Jax and his daughter Jacqui have variations that make use of Machine Guns. Jax having his signature machine gun in his Heavy Weapons variation alongside a Rocket Launcher while Jacqui's Full Auto variation gave her gauntlets machine guns and rocket launchers as well.
In Mortal Kombat 11, The Terminator and Spawn make use of machine guns in their moveset. With the Terminator using a single SMG and Spawn dual-wielding them. The Joker on the other hand makes use of a Tommy Gun in his Party Pooper Fatality.