The Hammer (Spoilers)

This one really exceed my expectations. I must admit that my jaw dropped when he hit the twins with that hammer. Most of the punches looked faked and I was waiting the same when they bring that thing up. But no. he first hit the dead one's head on the floor. I was surprised but thought 'The head may be a prop' then he hit the live one on the head hard. Was it just me that felt like it was real? What do you think they did to catch this effect on that scene? Rubber hammer?