Sort of a dumb question for the group:
I know that pierced primers are usually a sign of over pressure. If I rule that out (for example, indirectly with chrono data) and the piercing or flow is just from the soft Federal primer in my 9 mm loads, IS THERE ANYTHING DANGEROUS OR PROBLEMATIC FROM HAVING THE FIRING PIN PIERCE THE PRIMER?
Not a dumb question at all...it does happen.
Yes, two things can happen. The area around the firing pin becomes eroded by the hot gases and the debris the gas can carry can injure the shooter. If the pierced primer is not an anomaly but is the result of a too high of pressure in a batch of hand loads, stop shooting them. Pull the bullets and redo your load with a safer choice.