The Mec skill One For All allows a MEC Trooper to act as high cover for their meatsack comrades for the duration of the turn, provided the MEC Trooper doesn't use an arm-based weapon. This is deliciously vague, as is tradition for XCOM:EU skill descriptions. What exactly can and a MEC Trooper do without canceling One For All?
Which actions will cancel One For All? Which won’t
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Best Answer
The arm based weapons are, quite literally, the two weapons attached to the arms.
For the initial MEC-Warden, this means the Minigun, Kinetic Strike Fist, and Flamethrower will all disable One For All.
Reloading and overwatch do not end the effect.