Red Dead Redemption 2 Locked and Loaded Trophy Guide / Achievement Guide - Upgrade each available component for a single sidearm or longarm weapon. [Bronze]

Go to any Gunsmith (marked on map in towns), for example there's one in Valentine. Look at the Gunsmith with L2/LT and hold Square (PS4) / X (XB1) -- "Customize". Select any weapon -- "Components" -- Change every Component. For example the basic Double-Action Revolver has these 4 Components: Grip, Barrel Length, Barrel Rifling, Iron Sights.
It costs a few dollars to change these but you can make a manual save beforehand and reload it afterward to reset the money (Pause Menu -- Story -- Save Game / Load Game).