Weapons can be further upgraded through Raritanium. This currency can be dropped randomly from enemies or found within hidden stones across planets. The hidden stones with Raritanium can be found on the map via the Map-o-Metric gadget located at the end of the grinding sequence on planet Kalebo III.

When you have found Raritanium, you can use it at Gadgetron weapon venders by pressing  within its menu. Select the weapon you would like to upgrade, then select the type of upgrade (such as ammo capacity). Not all upgrades are unlocked at the start for a certain weapon, you will need to upgrade the weapons level to unlock all Raritanium upgrades. Once you have bought all available Raritanium modification upgrades, the trophy will unlock.