In "Rainbow Six Siege," the Obsidian weapon skin is available for several weapons across different operators. However, the specific guns that can receive the Obsidian skin may vary, as it depends on the updates and changes made by the game's developers. Generally, Obsidian is often applied to various primary weapons.
To find the most accurate and up-to-date information about which guns can receive the Obsidian skin, you can check the in-game store, the Ubisoft website, or community resources like forums and fan wikis where players often compile and share detailed lists of available weapon skins.