Here is the full list of all 20 MOTHERGUNSHIP achievements.
Add part to a gun 1000 times
Kill 1000 Spinbots
Find 15 secrets in a single mission
Build a gun that uses over 20 parts at once
Reach room 20 in Endless
Die from lava damage after killing a boss
Finish 1000 enemies using your fist
Equip 10 armor upgrades
Take 4 diceroll rooms in a single mission
Complete a mission without killing anything except the boss
Fire 65535 shots
Complete the campaign
Get 40 jumps in a single run
Survive 30 challenge rooms
Complete mission without taking any damage
Complete a mission without missing a single shot
Complete a nightmare mission without equipping any gun
Kill 5 mobs in one shot
Complete your first mission
Craft a gun with DPS of over 100