The location of every mighty shotgun that can be had in Mass Effect 3.
M-23 Katana: Starting weapon in the Priority: Mars mission.
M-27 Scimitar: Found within the main Turian camp, on the ground, during the Priority: Palaven mission.
M-22 Eviscerator: Found on the red couch during the Grissom Academy: Investigation mission.
M-300 Claymore: Found inside the left building within the abandoned camp during the Attican Traverse: Krogan Team mission.
Graal Spike Thrower: Found near the beginning of the Priority: Tuchanka mission, near the railing with the nearby guard.
Disciple: After you find the lone Asari during the Lesuss: Ardat-Yakshi Monastery mission, enter the combat room and clear it of cannibals to find this shotgun.
Geth Plasma Shotgun: Within the large combat area during the Priority: Rannoch mission, you’ll find this gun to the right side of the area, near the entrance.
M-11 Wraith: Can be purchased from the Spectre Office for 250,000 credits.