As a general rule, it's not the arrow, it's the Indian.
Situational awareness and the proper mindset > caliber.
That said, there are enough small "major" caliber guns out there that .380 in the pocket would be my minimum, but I've kept it to 45 or 9mm Shield for years without expending much effort or buying special cloths.
This^
I view defensive pistols a bit differently in retirement then when I was active duty. I carry a pistol to get me and my family off the "X" and to get to a better weapon. If I were forced to use my pistol there is a huge probability that it would be at extremely close range, maybe even contact shooting. Something to consider is that if you go after an active shooter and burn him down, in the eyes of the panicked people involved, any other citizen with a gun and the arriving police, you could reasonably be confused with the instigator.
As far as what is suitable, that's on you. I want whatever I carry to cause major cavitation in the thoracic cavity. Remember the goal is to stop the threat. No I wouldn't want to catch a 22 round but it essentially equates to harsh language to a 200# man wearing a hefty coat and full of adrenaline.
Just my 2 cents.
