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- In the Half-Life 2 Beta, the HMG1 is the only gun that uses heavy rounds.
- It has poor accuracy and can be switched to fire three round bursts. Strangely, each burst will only eat up one unit of ammo. It will continue to fire as long as the player holds down the fire key.
- It is very powerful, being able to kill a Combine soldier in only a couple of shots.
- The concept for an overwatch soldier  depicts it with the HMG.
- The Overwatch Soldier model has firing animations for this weapon in the playable Beta and the final version, another hint it was to use this weapon.
Behind the scenes
- An older HMG1 model named "v_hkgr9.mdl" can also be found. It is a much less detailed version of the HMG model.
- The number 1 in the HMG1's name hints that there was supposed to be another HMG, but there is no sign of it in the leaked files.
- No worldmodel for the GR9 exists. The pistol worldmodel was used as a temporary model at the time of the file leak.
- In the Half-Life 2 files, several texture files for HUD weapon icons can be found, most of which still containing the icons for almost all of the cut weapons. The HMG1 is among them.