"The left hand segment of an AMG-78 power-limited prototype."
— Item description
After completing the main game on Normal difficulty, the player is rewarded with the AMG-78a in every subsequent playthrough, and the item can be found within the Item Box at the beginning of the mode.
The AMG-78a functions very similarly to the original AMG-78 model, in which it has three levels of standard charging and thusly deals a lot more damage, with more range, with every level it is charged. As well, it is used by Joe's left hand and can be used in sync with his Bare Fists, though if done so the charge will be interrupted. However, it is somewhat weaker than its counterparts, being a power-limited prototype of the weapon.