You can purchase ammo/sights for your owned weapons from the Store in the Cache at any Outpost.
- Buy the ammo/sight from the store, then click on Storage and transfer it to your Inventory/Backpack.
- Exit the Cache and bring up your Inventory in the menu screen.
- Click on the rifle you want to add the ammunition/sight to and it will bring up a menu on the right hand side of the screen.
- You will be able to select the sight and the ammunition you want to use.
- Exit the menu and reload your rifle/you sight will be displayed.