The 9 mike mike is the most common round, it’s used by most miltary forces so running out will not be an issue. Same same for .223/5.56 and 7.62. I can carry more rounds of 9mm than .40. Heck, I can carry a brick of .22s in the space of 5 9mm/.40 mags. Weight matters when hoofing.
.22 may be small but most will break off an attack when hit a few times from max range. We are talking about people not Grizzlies.
Glock 32 and an M4. My back up is a .22 mag bolt action that is accurate out to 150yards+. I have 3000 rounds for it.
TSA runs the .40 so there will be warehouses of that around too.