I think a 4 bore double hammer gun would be a load of fun to shoot and hunt with. I consider hunting with a 31 pound rifle a stunt, since it is really not practical. But it is a stunt that is relatively safe due to the power unleashed when the trigger is pulled, assuming that the hunter can hit his target.

In that same regard, perhaps hunting with a .700 is a stunt because the H&H's weight 22 pounds and are not practical to carry either.

Probably the .600 is the upper limit since the guns can weigh 13-15 pounds and still be carryable.

To be 500,

I think the .500 jeffery, .505 gibbs, .500 A-square are below the line which specifies sensible & practical. I feel the .585 Nyati is sitting squarely on the line, maybe slightly below (there was purpose it it's invention, and you have provided evidence that it can indeed be a practical African rifle) and those calibres over are into "the realm of toys and stunts"