Woodleigh Hydro solids (and Swift"s new solid) are half way between a solid and a hollow point. The theory is the cupped point will generate a pressure wave ahead of the bullet, dealing additional flesh damage.
North Fork also makes a cupped point solid. But theirs is a bit more like a true "hollow point" bullet.
Again though... I dont know why there is so much discussion in this thread. You do not get to pick your projectiles with a double rifle like you do with a bolt action rifle. Anyone with experience loading for doubles will tell you, they are very picky when it comes to bullet selection. The gun will only truly regulate with the bullet it was originally built for and perhaps other very similar bullets. If the gun was set up with traditional solids than it is unlikely it will work properly with more modern brass/copper bullets.