The Heavy Machine Gun, also known as HMG1, was a weapon cut from the early stages of Half-Life 2. It can be found in the leak files.
The Heavy Machine Gun was meant to be used by the Combine and Rebels and the player would've been able to use it. It appears to be loosely based on the real life H&K GR9. It can also be noted the retail Combine Soldier animations still have leftover of the firing animation.
The weapon was cut for the final release, probably due to the fact the amount of weapons that existed at the time.
Functionally, the weapon has horrible accuracy and a medium-low firerate, roughly the same as the pulse rifle's from the retail HL2. It was two firing modes, the primary mode being automatic and the secondary mode being a 3-round burst.