First, navigate to: steam\steamapps\common\IL-2 Sturmovik Cliffs of Dover\manual
Open the .PDF file containing the manual in your language of choice (there are 6 .PDF manuals, each in a different language) and look for the following sections. The page numbers are for the English manual, not sure if they are the same for the others.
- Loadout - weapon sets (page 54): explains how the weapon trays are loaded/removed in the simulator (you can fly an unarmed aircraft if you like, or remove some guns to save weight, etc).
- Loadout - guns (page 56): Explains how to load customized ammunition belts
- Common ammunition types (page 58 ): Explains the different ammunition types.