I just looked at some pictures I have of the muzzels of my [new to me] Purdey 10 bore which the factory letter says was made for ball. There are 12 lands and grooves, the grooves look to be 2X the width of the lands, and are fairly deep [.005 or so]. The rifle was made in 1896. As I remember, the twist is about 1 turn in 30-36 inches and uniform. My preliminary shooting with it is very encouraging with 5" 50 yard groups.
All the best, Mal