There is some ambiguity in the Tearing property.
The Dark Heresy Rulebook states that:
These weapons roll two dice for Damage and choose the highest.
Rogue Trader, Deathwatch and Only War are agreed that:
These weapons roll one extra die for damage, and the lowest result is discarded.
For most weapons it's quite simple because of damage like 1d10+something. But what if we have more dice in the damage roll? Should I roll an additional die for every base damage die, or only one for the entire damage roll?
For example, the Angelus bolter from the Dark Heresy supplement Inquisitor's Handbook has 2d10 damage and (like all bolt weapons) the Tearing quality. Let's presume that my character shot a heretic with this bolter, and now I must determine inflicted damage. Should I roll 2d10 twice, and discard lowest in both, or I should roll 3d10, and discard the lowest one?
Tearing weapons roll an extra dice in addition to however many dice are required for the weapon and then the dice with the lowest result is discarded.
For this weapon you would roll 3d10 and discard the lowest.
Bob the Marine shoots Eric the Heretek with his Angelus bolter and hits. This weapon does 2d10 tearing damage. Bob's player rolls 3d10 to determine damage and gets a 3, 5 and 8. The 3 is discarded making 5+8 = 13 damage. Eric is going to be unhappy.