In this guide, you will find out how to unlock the barrels in the crafting station on Mars. Go through the guide and follow the instructions carefully to do so.
You have to approach The Relic and reshape your pulse rifle. Click on Revision Zero and navigate to Hunter’s Trace on the left-hand side.
It is how to get Hunters Trace two. It says “Unlocks additional barrels for shaping. Acquired by completing operation Seraph Shield at any difficulty setting”. Go back to complete that mission and then you will have full access to Hunter’s Trace2, and you can change your barrels.