If this is something YOU want to do, then I'd look into the .22 Squirrel, based on the shortened .22 Hornet case. It should work well for down loading as you describe, and equal or slightly exceed the .22 WMR when loaded up. However, for a "bug out" gun, I am not sure why you want a single shot? That situation would be conducive to MUCH disorder, where single shots COULD get you killed. Just MHO, though.
