If you want to get the concussion gun free you need to follow these steps
STEP 1:(the most important step)DO NOT BUY THE CONCUSSION GUN!
STEP 2:Wait until the level dream time(the 1 when ratchet is asleep)
STEP 3:Complete dream time
STEP FOUR:When you are on medical outpost omega your weapons will be split out through out the level but the creators of this game would expect you 2 have the concussion gun and it will be lieing on the ground some where through the 2-3 hallway