Simply fully upgrade a Bow, Handgun, Rifle, Shotgun, or Axe at a Base Camp to unlock this achievement. The Climbing Axe is the easiest weapon to fully upgrade and you’ll only need to find the Crafting Tool and Enhancement Tool.
The other weapons require you to also purchase the Refinement Tool for 175 coins from the Supply Shack. The Crafting Tool is in a strongbox in the Abandoned Mines and the Enhancement Tool is in a strongbox in The Acropolis.