By asking questions and doing your research. Sometimes a difficult task, unless your aware of Nitroexpress. Mick is not asking for the moon with his Holland 8, its probably priced accordingly but can you live with its faults?

Rigbym,
True big bore made for DG smoothbore ballguns seem to be quite rare. I have only seen 4 including the Holland under discussion that by their configuration (length weight and charge) and documentation (factory records) could be described as, ball guns, elephant guns etc. Made specifically for close encounters with angry animals they need not shoot as has been stated earlier more than 60yds but with more velocity and less recoil than a fully rifled gun.
Big bore paradoxes aren't a compromise. The dirty little secret about the big paradox guns is they are very accurate and easy to regulate even with a little wear in their bores.
Another good read is Sanderson "Thirteen Years Amoung The Wild Beasts of India". My avitar is the business end of a ballgun as described in the book.