A Ruger MKIII. I can easily hold 500 rounds in my pockets--try that with a 12 guage, .223 or even 9mm. You may get a look from the shtf ladies running around though. I don't think there is a better shtf gun than a .22lr pistol. You aren't going to be taking 200 yard shots at anything.
A .22 is a good choice as well! Shouldn't have ruled it out. But im not walking anywhere if shtf, so idc bout 500 rnds in my pocket. The back of my suburban will be loaded down and ill be headed to south ga where the rest of my redneck, gun loving, paranoid family is! lol