[Zzvukttz]

With shinken, much of the cutting power comes from withdrawing the blade more than the cut itself. Given this, I'd suggest it would be extremely difficult to penetrate bone with a seme men, but you'd easily cut down to it. When it comes to killing someone, the top of the skull/forehead is actually an extremely inefficient location to strike (The most efficient I would assume being kesa to the shoulder or neck). I've had instructors tell me that the reason men is practised is because it is far more difficult than any other cut.

If you're set on penetrating skull though, I'd suggest it depends on the strength of the person. The idea that cutting requires no physical strength tends to be idealised in JSA, but when you're cutting through bone it's ludicrous you'd be able to do that without significant strength to back it up. I'd say a large, strong kendoka might be able to cut through a significant way, but a smaller practitioner wouldn't be able to do it using a small cut