Bill Wilson started a thread this morning, with some very interesting info on his survey of trainers and their ammo choices. In that thread, DocGKR said the following:
"Current 9 mm is the best choice for the vast majority of defensive uses--it is what I carry; the only exception might be in jurisdictions that have an illogical magazine capacity limit of 10 rounds or less, then some consideration might be given to going with .45 Auto. Also, if in a predominately wilderness area, .40 might be a reasonable option, although on my recent 100 mile backpacking trek through the Sierra's, I just stuck with 9 mm."
During part of the winter, I recreate above a state with a ten round magazine limit. Given gravity, it is possible that I might be in a ten round state. So, if limited to ten rounds, what is your caliber choice. I realize this is interrelated to what pistol you carry. For example, the Glock 17/19 does not have a stellar reputation with neutered magazines. Generally, caliber gets balanced with capacity, but here ten is the limit.