For just pure fun and economy the .500 S&W is a great way to go. I have a 4" Smith and 20" Encore and both are a hoot to shoot...
I've settled on 350 grain bullets as they are inexpensive and don't recoil much even with full loads...
I once shot a friends 500 double rifle...it was the only gun I shot once that I was not sure I could/wanted to fire the second round...but I did...just no joy in it at all...
Bob