First, please excuse me, I have not read any of the previous posts, just came across the thread. I hope I have not stepped over anyone.
Just going off the title.
What is the lowest caliber you will carry
An answer, with an explanation. A 9mm Parrabelum, loaded with proven rounds, that has been carried by a group, such as a federal, or State/City Police force. a group large enough to have seen the affects, of this round, on dead felons.
In my case, 147g Ranger non Plus P. Sixteen of them ready to go, in a Glock 19.
This combination is controllable, and will reach deep enough to reach major organs in a human torso, quite accurate, so that that precise shots can be achieved, and rapid repeat rounds, if or when desired, are doable.