You need to have a point of damage left over after the Damage - TB - AP equation (modified by things like Penetration and/or Felling) for the attack to deal Magnitude damage.
So...horde has TB 4, Armor 4. You hit it with a 2d10 Pen 0 frag grenade. You need to do at least 9 damage to hurt them (9 - 4 TB - 4 AP).
If you're instead shooting the same horde with a 1d10+9 Pen 4 bolter, you'll automatically damage it (minimum 10 damage - 4 TB = 6 leftover automatically, AP fully negated by Pen).
That is what I thought. At least 1 point of damage left over. This simplifies things greatly.