Shadow of the Tomb Raider
99 Achievements
1,690
47-64h
Makeshift Arsenal
Fully upgrade a weapon
15
23.95%
How to unlock the Makeshift Arsenal achievement in Shadow of the Tomb Raider - Definitive Guide
Segmentnext's guide on where you can acquire all the weapons also details exactly what items you'll need to upgrade your weapons.
https://segmentnext.com/2018/09/14/shadow-of-the-tomb-raider...
For most of the better upgrades of your weapons and gear you'll need some rare materials, outlined in this article by Gamewith... you should be able to easily find everything you need with just a little bit of grinding and elbow grease.
https://gamewith.net/shadow-of-the-tomb-raider/article/show/...
The skill for level 4 upgrades costs 2 skill points, plus the skill points required to unlock an adjacent skill.
Before you can fully upgrade a weapon at a base camp, you must first unlock the Warrior skill called Raposa's Wit. This skill will allow you to craft additional upgrades for every weapon type.
The easiest weapon to fully upgrade is the Recurve Bow, as it only has eight upgrades. Take note, however, that upgrading weapons will require a fair amount of resources (animal hide, rare animal hide, salvage, wood, etc.). As such, it's extremely important that you loot any and all lootable resources over the course of your playthrough. If you have the funds and would like to purchase crafting resources, then you can do so at any merchant in Kuwaq Yaku, Paititi, and/or the Mission of San Juan.
To fully study a weapon, you will need 1 skill, which increases the level of weapon improvement.