The MP7 is an automatic PDW (personal defense weapon) developed by Heckler & Koch, designed as a supercompact armor-piercing submachine gun (SMG). It fires H&K's proprietary 4.6x30mm pistol round, a round used only by it and the MP7's pistol equivalent: the H&K UCP.
SMG vs. PDW
The 9x19mm Parabellum cartridge has existed for over a century and is used in a wide variety of pistols and submachine guns, offering respectable power for a case of its size. Unfortunately, with the advent of lightweight body armor, the old Parabellum is underpowered in situations where the enemy might be wearing kevlar; this type of ammo is so ubiquitous that Class II body armor is specifically defined as being "9mm FMJ-proof." PDWs are a recent designation for pistols and SMG's built to penetrate lighter forms of body armor, an effect generally achieved by using newer and more advanced bullet designs. The 4.6x30mm HK is an example of such a round.