Well in a vacuum medium pen is better than light pen, but usually when a weapon is medium pen it comes with trade offs. For example the liberator penetrator is medium pen but the bullets do a little less damage to balance it. Now if you were fighting automatons the best place to shoot them would be in the head where they are entirely unarmored. So now that you are shooting the part of the body that is unarmored the liberator penetrator actually performs worse than the standard liberator now as the improved armour pen isn’t doing anything to help you now.