The full version is generally a paid product available on Steam and other digital storefronts.
The developers occasionally host "Free Weekends" where the full game can be downloaded and played for free for a limited time.
Sandstorm: “Operation: Overrun,” New Major Update Now Available
It is important to distinguish between free updates and the full game download:
The game you are likely referring to is , a tactical team-based first-person shooter. While the game itself is typically a paid title, it frequently offers free content updates —often titled "Operations"—to all owners of the base game. Understanding "Free" Availability