Your weapons will sometimes get dirty and less effective in Red Dead Redemption 2. To keep them in good working order you’re going to need to fix and clean them from time to time. When you’ve been using a weapon a lot it will get dirty.
You can clean your weapon by opening the weapon wheel and selecting the weapon you want to maintain. Once you get to the weapon you want press the right thumbstick in. This will open a screen where your character is looking at the weapon and can if the weapon needs cleaning or not. It will also show you the stats that will be gained by cleaning the weapon.
On the right side of the screen you will see the option clean with the X/Square Button.
Pressing that button will use Gun Oil if you have it in your inventory. Cleaning the weapon with the oil will restore its stats.