"What caliber fire arm do you open carry,and why?"
10mm. Because .460 Rowland is too much gun to edc, and because factory .45 Super is loaded too low for the cartridge size, and because .40 S&W can be improved upon a lot by making the cartridge just a little bit longer, and the gun just a little bit heavier and bulkier.
If I were reasonably certain that my first shot would hit CNS, I would carry 22.
If only we all were that accurate!
But, I am reasonably certain that my first shot will have reasonable chances of only hitting the periphery of the target...
Really? How so?
...so I need something that will deliver more than ho hum psychological stopping power.
I'm fairly certainly that pretty much everything from a .380 ACP to a .44 magnum delivers roughly the same degree of "psychological stopping power" in the form of widely varying but equally alarming sharp reports.
The 10mm cartridge is the largest cartridge that when loaded to efficient pressure, is not too much for the standard duty autoloader.
What do you mean by "autoloader?"
If you're referring to the firearm itself, then pretty much every semi-automatic firearm from a .22 to a .50 is capable of handling the round it's designed to handle.
If you're referring to the device capable of loading magazines, well, then again, devices are made to fit a range of calibers.
Glock 29 with 15 round magazine with upper-end Underwood is what I think is the optimal balance between terminal effectiveness on one hand, and ease/comfort of open or concealed carry on the other hand. I do not have license to violate the laws of physics. If I could pay a lot of extra money to get 460 Rowland performance from a gun that is smaller than a G29, I would plunk my money down.
That depends mostly on the shooter. There was a time when I thought nothing of firing my .44 magnum. Then, one day, my wrist ached for a couple of days after the range. Now I'm a fan of 9 mm. I have a friend who can fire a .50 single-handed without blinking. Another, much more diminutive friend, has difficulty firing a 9 mm, so she's opted for a .38 with special loads.
So far, so good. 25 months of walking six miles per day equals 4,560 miles and zero minutes of hospital time. Less than 24 hours of legal trouble time.
The actual statistical odds of encountering a situation where one might reasonably need to use their firearm is roughly even (50%) in about 37 years. These odds do, however, vary wildly depending on your location and excursion habits.
The only reason why I carry is because when I need to stop a threat immediately, I can potentially stop a threat quickly enough. The only reason why I open carry is to save critical draw time.
I carry because I'm a bit too old to run. I open carry both for deterrent value and rapid employment. I conceal carry in situations where the threat is low and it's important for me not to alarm others as it tends to disrupt the event I'm attending.