I would advise against using small head nails. Hardi is tough counter-sinking wise Craig so my suggestions would be: 1) go to a store that sells roofing materials (not Lowes or Home Depot) but a real roofing material store or possibly a "real" hardware store and get the HDG roofing nails. Or purchase a countersink bit for the screws. That you can probably find at a big box store.
