What I don't understand in particular are these two phenomena that would seem to reduce the effectiveness of the applied mass:

1) when the deadblow is accerated toward the object the mass (majority) is in the rear of the cavity (from inertia). After the deadblow makes contact the mass leaves the rear of the cavity and travels to the from and as it does, it decelerates. (Loss of a thus loss of efficiency)

2) there is still bounce back inside the deadblow head. After the shot is thrown against the front inside of the deadblow it will bounce back. The energy that is lost to internal bounce back should equal any energy lost to the bounce back of a non-deadblow mallet of equal mass. Correct?