9mm.
-Self Defense
-Plinking
-Training/Classes
-Lots of great pistols in 9mm from subcompacts to full size. Even a few revolvers.
-Cheaper than any other major caliber from .38, .357, .40, or .45.
- Ammo is gonna get more and more expensive as the years go on. The 9mm will remain the cheapest and easiest to find.
- Can take small game up to small deer if I was forced to hunt for game with a handgun for survival (not likely anyways).
- When I get much much older with achy joints, sensitive to loud noises, and even more grumpy and crotchety I'm sure I'll appreciate the easy shooting qualities of the 9mm over .357 or .45.