We may just be going around in circles. On most of the ironclads, the most dangerous item might have been flying bolt heads or nuts. This would have varied from vessel to vessel, depending on how the armor was attached. I seem to remember reading that the turrets of the monitors at Charleston really took a beating and had bolt and nut problems. The Albemarle probably didn't have this risk as both ends of the bolts securing the armor strips were flattened. They didn't have the machine to do the threading, so they just did without the nut. This was pretty practical unless you took heavy damage and had to replace the plate. Arkansas also took some heavy fire, much at close range. Her rail armor held up fairly well, but I believe the casement was pierced at least once at Vicksburg. If might be useful to reread the casualty reports to see how they were caused. Other casements that took a beating were the Tennessee II, Fredricksburg, and Atlanta.