My take on it has always been that the Hand Cannons in the world of Destiny came about as a result of fascination with pre-golden age machinery and technology. They work like present day firearms, but with completely different inner workings; hence the massive hitting power and unrealistic ammunition capacities in some cases. Also, and others may have pointed this out, the Chiappa Rhino revolver has a barrel aligned with the low cylinder and a (false) hammer aligned with the high cylinder. This may have been a source of inspiration as many of Destiny's hand cannons also feature hexagonal cylinders.