All of the Weapon I Challenge achievements are quite similar. For this one, you need to kill a total of 50 enemies with 3 different variants of the Autocannon.
You can track your progress by pressing
, then browse to
Challenges -> Weapons -> Autocannon. Select the rifle you want to track and hit
.
The autocannon challenges required for this achievement are:
Torrent I
Cloudburst I
Mauler I
Basically, any mob can drop them. You will also get it off treasure chests and other mission rewards. Also, note that this is a Colossus-only gun so it can only drop when you are playing the Colossus javelin.