To elaborate, unless a rule states otherwise, when you score an additional hit on a roll of 6 (or 5+, or 4+, or whatever) it's considered to be a hit with no assigned value.
Likewise for a bonus wound on a roll of X+.
So, a 6 to-hit with both those rules would give you:
-A 6 to-hit, that wounds automatically.
-A bonus hit, with no assigned value. So, roll to wound with it normally.