.$ac is Autodesk AutoCAD autosave
Key Features: .$AC File Extension
What is an .$AC file?
The $ac file extension is associated with Autodesk AutoCAD, a popular CAD modeling software for Windows and Mac.
The $ac file contains autosaved CAD drawing. It is used as a backup of AutoCAD project against application, or computer crash. After the project is finally saved by user, the autosave file will be deleted.