NAA Mini Revolver, .22WMR, 1 5/8" barrel. I load it with Federal 50 grain HPs, but there are now many high-performance defense loadings available. Fits perfectly in a nylon single magazine pouch ($5 at any gun show), disappears in my front pocket. Never, ever leave home without it.
On the one occcasion in the last 20 years I had to "brandish" it, the impression made on the other 2 "gentlemen" was sufficient to save the day.