The MP5 has a ton of variants and each can be modified.

In games usually you find a submachine gun close to the MP5 A4 or MP5 A2, if a game just says "MP5" we reference to the MP5 A4 then.

The variants use the calibers 9x19mm, 10mm Auto (for MP5/10) and .40 Smith & wesson for the MP5/40 gun.