Depending on where you hail from, average black bears can run from 200 pounds on up to 5-600 pounds, some, even more.
These heavy weighs oft times outweigh inland grizzlies, so I suggest .54 as a good starting calibre for both. Now, a .45 will
kill both species quite handily, however an outsized critter will "carry" considerable fat and muscle - going larger is a good idea
I think.
.54's make nice rendezvous calibres as well. The "components" for about any standard calibre are easy to come by, and moulds
of .530" to .535" are also commonly used for .54's.
www.trackofthewolf.com has everything needed. There are also other outlets.
Welcome to the forum, Jack.
To answer your second question about 'gatherings', I would suggest perhaps .50 is the most common calibre, but that might depend
location. Up here in Canada, .54's are REALLY popular, but I suspect in and around Tennessee, .40 might be very popular.
My own personal hunting rifle is a caplock in .69. It is also my "target" rifle for rendezvous.