Clarity of the optic would be best if not flattened but rather allowed to cool from a molten state. When heating do not allow the flame to contact the fiber which contaiminates it, also reducing the clarity. You may want to try this: In a thin piece of metal, drill a close fitting hole for the fiber optic to pass through then counterbore the hole. Heat the fiber then lightly pull it up flush to the metal which will form a shoulder and produce a bulbous end. You probably won't get the 3mm length with this method but it would be a bright and clear sight picture.
Happy shooting,
Charles