|It looks kind of rusty|
The Mach-9 is based on the German H&K MP5 submachine gun which is chambered for various handgun cartridges, NOT intermediate rifle cartridges such as the 5.56mm. It's one of the most powerful, and popular, guns in Nether.
How to obtainEdit
You can obtain a Mach-9 by putting one together, or by killing the Reaper. You can trade various items with the Order of the Cull to obtain the rifle frame, 9mm barrel, and the 2x scope. The fourth, and final part, can only be dropped by a Nether, or found somewhere on the map. It's called the burst mechanism, used in burst weapons.