Genestealer Claws are razor sharp, able to shred even Terminator Armour. Any attacks from Genestealer which roll a 6 to hit will wound automatically and ignore armour saving throws. Other attacks which hit on a roll other than 6 must roll to wound as normal and the victim receives their normal armour saving throw. When rolling to penetrate vehicle armour, if a 6 is rolled then the user can add an additional D6 to the score (thus giving Strength + 6 + D6 in all). Both Genestealers and the Patriarch are armed with Rending Claws as noted below. Note – having Rending Claws on any model (Genestealers, Patriarch, etc) does not give them an additional attack, only what is listed in the profile.
Rending Claws
Source: Codex: Genestealer Cult, p. 5