I can't get a good grip on the Beretta either, and as far as the .500 Smith is concerned, if I can't do it with a .44 or .45 I'm not sure I want to try.
I think quilla's husband has a .500 if you want to find out what it's like.
Having been in artillery, my bias runs towards 'bigger boom'.
I'd wanted to try my hand with an H&K USP .45, but it's grip was about the same size as the Beretta's.
If I could get something in that size range, I'd probably have to work on wrist strength to be able to hold it steady.
(Not something one usually has to worry about with rifles.)