The machine gun is an automatic weapon.
Zombotron 1
In Zombotron 1 the machine gun is first available on level 8 in the store. It has a 40 bullet clip in Zombotron 1.
Zombotron 2
The machine gun is not available on Zombotron 2 , but on the last level the scientist is using it to kill the player.
Zombotron: Time Machine
In Zombotron: Time Machine it is available for purchase on the last level,but it is found in a golden chest in level 8. In Zombotron: Time Machine it has a 60 bullet clip.