Hellfire Torpedoes
From FEInfobase
Hellfire Torpedoes, were a failed Starfleet experiment to make a torpedo capable of skipping past an enemy vessels shields, although initial testing looked promising further development showed the torpedoes to be highly unstable when lauching and more often than not they phased all the way through a vessel before exploding harmlessly beyond it.
Coupled with the large quantities of resources required to build a single Hellfire Torpedo the whole project was shelved.