The chain I am willing to repeat
The guide sends you through the Titan City barrel escort toward a yellow platform and a red gate, then into a side building and a downward parkour. Two laser bosses guard a Broken Frame and the Server Access Codes. The codes open the hub server room above the drop pod. That room is module one.
The four module ids in that guide are 1A7B in the hub server room, 89J2 in a desert crash ship at the lower-left thruster, 88B4 on a shelf under a tundra building with two waste pipes, and 2A30 on a high fungus in the Gravity Farm. Two Saxonite each, eight total. After 1.9 the guide says modules cannot be scrapped. Diamond is the layout named by two sources. Another site says triangle. Fit them to the grid you see.
What the gun is, once it fires
Miraheze calls it a melee chainsaw and lists 21.85 damage, 526 RPM, a 100-round magazine, 200 reserve, and a 3 second reload. I have not put a stopwatch on 2.0.2. The patch notes mention Carver Sharing Lunch, so something on the gun moved.
If you only want the last locked guns, Globbler is the other famous one and Accelerator is the third. They are not modules for this chainsaw.