Would this need to be classified as a magic item?
I mean other than Size, I don't see a difference between "Runic Bullets" and real world "Custom Shotgun Ammo". Look up Dragon Fire Rounds, Bolo Rounds, or ball and shot. A weapon smith can customize shotgun ammo fairly easily.
Imagine a player, who before treading into a dangerous situation, makes a lore roll to discover the weakness or "Catch" of the target. Then while locking and loading, makes a resources roll, to place an aspect of "Customized Ammo" on their character.
Would this not do the same thing? Or, by introducing "Runic Bullets" are you simply giving the players a shortcut, by which they do not need to know the weakness of the creature, they just shoot it with the provided ammo till it goes down?