Fistful of Frags is a Free-To-Play Source Engine complete modification of an old Western shootout first person game. It consists of Old West themed levels & weapons with your typical deathmatch, capture the flag and team deathmatch styles of gameplay. There is even co-op levels with bots.

This game has been around for a while, but still has an active community. In fact they released a Linux version too.

https://store.steampowered.com/app/265630/Fistful_of_Frags/