But you still have deceleration. A point that gets at the increased efficiency claim. The magnitude can certainly be debated.

I'm not sold on the increased efficiency from a better recoil property for the shot. It's still going to recoil. Bang into each other (energy loss) and bang into the sides of the cavity (energy loss)

You also have a loss in energy via heat (from banging into each other) that doesn't come into play on a solid hammer of equal mass. Again, this gets to efficiency. Magnitude can be debated.

I don't think we get a free lunch here. More efficiency from a hammer strike with the same amount of mass - not convinced.