In order to unlock the Lachmann-556, you need to progress the Lachmann-762 to Level 13. To level up a weapon, you need to use it often in multiplayer matches in order to gain proficiency in it.

Players can unlock the STB 556 at level 41. Additionally, leveling the STB 556 to level 13 will unlock the MX9 and leveling the STB 556 to level 20 will unlock the HCR 56.