History of But This One Is Mine

But This One Is Mine is a triumph worth 25 points: "Gain 20 weapon levels using crafted weapons." Weapon levels can be gained across several weapons, so they add up between guns.

How to complete

1. Craft one or more weapons at the Enclave using a pattern (patterns come from Deepsight Resonance extractions and some quests, raids and seasonal activities).
2. Equip the crafted weapon and use it. Per the guides, crafted weapons level up by defeating enemies with them and completing activities with them equipped (TheGamer, Shattered Vault).
3. Keep leveling until the combined total across your crafted weapons reaches 20 levels.

Tips

• TheGamer recommends kill farms for fast XP, for example the Shuro Chi area in Last Wish or the opening area of Grasp of Avarice; objective-based activities also give a lot of XP.
• Use crafted weapons in activities rather than idle patrol for faster progress (Shattered Vault).
• Since levels count across weapons, you can rotate several crafted guns instead of grinding one to a high level.
• Weapon level is separate from Power level. Source guides disagree slightly on thresholds (maximum customization around level 20; mementos at 30), and these may have changed since The Witch Queen, so check in game.

Rewards

Triumph score only; no other reward was confirmed in the sources I opened.