True Renaissance Fair cosplayers, true historians, true Internet TRUEfans know there is only one superior medieval firepower technology: The trebuchet. As opposed to the crude rubber-band twang of a catapult, with its need for high tension and so many points prone to failure, a trebuchet harnesses basic physics in the most practical way.
This can also make a cool, even functional, miniature for tabletop gaming, should that be an itch. When your siege engine needs to absolutely, positively get that ammunition through express overnight delivery, choose the superior medieval technology. Trebuchet, from the sadistic twisted whack jobs that brought you the guillotine.