The LifeCard - a .22LR credit card-sized single-shot pistol. From Trailblazer firearms. Admittedly thicker than a credit card, but it fits in an Altoids tin, which is a common DIY form factor. $400.
1:02 video:
https://www.youtube.com/watch?v=GZ4JcZc6PsI
https://diqn32j8nouaz.cloudfront.net...ni_pistol2.jpg
https://diqn32j8nouaz.cloudfront.net...ni_pistol3.jpg
https://diqn32j8nouaz.cloudfront.net...ni_pistol4.jpg
https://diqn32j8nouaz.cloudfront.net...ni_pistol1.jpg
More: LifeCard .22LR | Trailblazer Firearms
Previously:
https://www.homemadetools.net/forum/...877#post105666
https://www.homemadetools.net/forum/...794#post119004
https://www.homemadetools.net/forum/...250#post112988
https://www.homemadetools.net/forum/...230#post116025
https://www.homemadetools.net/forum/...5576#post82133
