From The TimeSplitters Wiki

The Machine Gun is an SMG with a very high RoF. In contrast, the accuracy and power are relatively low, especially in comparison to the K-SMG or the Soviet Rifle. The Machine Gun has only one firing mode, but it can be dual-wielded.

It only appears in TimeSplitters: Future Perfect. It makes its first story mode appearance in "The Russian Connection". It bears a very obvious similarity to the Heckler & Koch MP5K.