You are doing absolutely nothing wrong. This is rather the norm when lapping the .444 Marlin. Two things here enter into the picture to cause the stiff closing of the action. Number one, the bullets are oversize, to insure that you are getting the necessary lapping action at the bottoms of the grooves in your barrel. Secondly, the bullets are seated deeply to contain all of the bearing surface of the bullet in the case, thus making it appear almost as a wadcutter load when properly prepared with the bullet deeply seated. When the bullet is seated this deep, the bullet base protrudes into the case, to the point where the brass thickens very, very quickly in the case walls. Between these two factors, often times you'll have a case that is a bit on the fat side for a tight chamber. You're doing no harm to the gun or it's chamber by going ahead and stiffly closing the action on your lapping load. The loads are such low pressure rounds that any extra resistance to the bullet in the case is inconsequential in regard to it's being squeezed a bit!
Perhaps this will ease your mind!