On this page of the guide to Sniper Ghost Warrior Contracts 2, you'll learn how to refill your ammo. This is quite important, especially at the beginning, due to the fact that you can miss the ammunition boxes. This section of the guide to Sniper Ghost Warrior Contracts 2 will allow you to remedy that.
Especially at the beginning of the game, when you have to get used to the mechanics, it may be easy to find yourself short on ammo, especially for the sniper rifle. One way to collect ammo is to search the bodies of defeated soldiers. While on short-range contracts, this is probably not a problem, on long-range contracts, it's much harder.
The second way is quite simple - in long-distance contracts, you will find spots suited with stuff you can use, including boxes with ammunition (as in the screen below). Note, however, that you can only use them once. The boxes also contain explosives, booby traps, as well as grenades. It's easy to miss some ammo and ammo crates - use the mask mode and search soldiers to avoid that.