This sounds like a question from someone who has not cast many bullets. The bullet mold sprue cutoff is at the base of the bullet. You can't insert anything hard through the hole because it would ruin the sprue cutter. If you just drop something in the mold, it might point any direction, at complete loss of accuracy. Keeping the whatever centered would be a big problem.
It would be easier to drill a hole in the bullet and insert the "thing".
Now, what a prosecutor or plaintiff's attorney would say to a jury if you ever did use it on anyone...???