Unknown to many historians Tokugawa used this bottle against Toyotomi in the siege of Osaka Castle. Tokugawa hid 2000 Ninjai's ( www.ninjai.com ) inside and well you know the rest of the story.
Damn you Tokugawa and your Budweiser!!!!
Sorry, our family was under Toyotomi, I get a little emotional sometimes.

On a serious note, I believe the Kabutowari was a legitimate weapon,
HOWEVER I do not believe it was for smashing helmets. It may have received the name because it could crack a helmet, but lets be logical. You would have to be right next to the person and smash him on the head with extreme force without him attacking you. You would be quite vulnerable. It would be best used running into the Samurai locker room and smashing helmets before a battle. There are many decorative, but still usable Kabutowari out there which were probably made in the Meiji period, these I believe are geared more for display / export.
What most of us see is a Tekkan which looks like a Kabutowari. These were the answer to the Haitorei Edict which abolished edged weapons, the Tekkan were absolutely legal to have as they had no edge.
Just an opinion.