I saw this video where a professional card thrower showed some people how you can use a rubber band to "throw" a playing card with surprising accuracy and power. I decided to design this simple his simple slingshot to keep the rubber band from slipping, making it even easier to fire playing cards. The slots are designed for a size 64 rubber band. While this thing safe enough for indoor play and could be considered a toy, this should be treated with caution and care. Never fire directly at someone's face or in their general direction. I couldn't embed a video of it in action since I don't have a Youtube page, but trust me, it works really well.
Assembly and "firing" instructions here: https://www.instructables.com/id/Playing-Card-Slingshot/