The hellbore cannon was a Hydran heavy weapon, similar to the Federation photon torpedo, with a few differences: the warhead had a force field that activated upon firing. Also, the warhead was likened to a fusion bomb and the effects of the force field caused the fusion explosion to wrap around the entire ship then implode. Due to the nature of deflector shields, this would cause the greatest damage to the weakest shield.
Also, hellbore rounds interacted with expanding sphere generators in a bizarre manner: any Hellbore that crossed the path of an ESG, even if targeting another ship, would immediately damage the ESG field. If the ESG field collapsed, excess damage would strike the ship generating the ESG. (Star Fleet Battles)
The RIS Bouteina was hit by hellbore cannons on several occasions, the first of which was when children afflicted with the child-king syndrome opened fire on the Bouteina because of their caprices. (RIS Bouteina: "Spoiled Rotten")