Bill,
Are you measuring firing pin protrusion from the breech face, with barrels off? Measure the protrusion with the barrels on, both unfired and after dropping the hammer. A half inch rod works. I would say your on the right track, something is either slowing down, or limiting the hammer fall. The forend iron rotates the crank on the cocking rod,in cocking, and then it rotates the crank out of the way. Is the crank on the action end on correctly?

Dennis Potter