FINAL-EDIT: As of August 2023 There is now a workaround if you are having issues getting this to pop that should force it to unlock. The Method is as follows
- Player 1 (going for achievement) joins an online co-op game hosted by Player 2. Join using your primary character.
- Play for a few minutes until the game has saved. Easy to do if Player 2 progresses a mission or if you go in/out of an interior level.
- Player 1 leaves the co-op game via the menu. Time since save is displayed in seconds, which can help you check you’ve had a recent save.
- Player 1 loads up single player like usual with their primary character.
- Go to a gun shop and buy a weapon you don’t already have (this is probably optional, but why chance it?)
- Go to a vendor to upgrade the weapon. Upgrade it from lvl 1 to 10, or until achievement unlocks.
All credit goes to Hirnforscher for discovering the method:
https://forum.psnprofiles.com/topic/145563-how-to-still-obta... and to terrett101 for bringing that info onto TA.
EDIT3: As of August 2022 this achievement still appears to be buggy, with numerous people stating that they did not get it to pop after fully upgrading a weapon. With the new dlc Cyber Heist out: There appears to be a bunch of superior components available in it which will make it easier to upgrade for those struggling to find those in the base game. Unfortunately if you are glitched out of this achievement the best option still appears to be creating a new character or deleting your save and restarting fresh.
EDIT2: This achievement is currently very buggy and you may struggle to get it to pop after upgrading your weapon. I was able to get it to unlock by upgrading the first weapon that I spent components on (I upgraded other weapons during the game but I never let those other weapons go to a higher level than the first weapon I had upgraded). Cisco14 also mentioned that they were able to fix the bug by creating a new character and getting a level 10 weapon on there so if you can't seem to get this to unlock that may be worth trying.
Original Solution:-------------------------------------------------
Within the hub areas where all the vendors are is a "Gunsmith vendor" (The Hammers on the minimap), here you can upgrade any of your weapons if you have the parts ready.
There are 3 different parts that are used for upgrades: Basic, Advanced and Superior and a total of 10 upgrades needed.
from level 1 to 5: 1>2>3>5 component I (basic)
from level 5 to 8: 1>2>3 component II (advanced)
from level 8 to 10: 1>2 component III (superior)
You need to complete the Basic component upgrades before doing the Advanced upgrades and so forth.
You can find all of the components by exploring areas of the game, they're a purple item and usually found in the paths least taken. Basic and Advanced parts are also sometimes dropped by Bounties once you defeat them.
Superior parts are
very sparse throughout the game so I would recommend spending your first 3 parts towards a weapon to upgrade it to level 10 for this achievement rather than spreading them across 3 level 9 upgrades.
One last thing to note is that these upgrades are permanent, if you accidentally sell the last version of a gun the upgrades will remain once you pick up another one!
edit1: fixed solution thanks to F77 and whyatt for the updated info!