Lets put this topic where its supposed to be:
How does Horde Damage work?
1 per hit + 1 per two DoS
Additional +1 if you are using a power weapon
1 per hit, +1 if you are using an Explosive (X) Weapon
1 magnitude damage per Blast (X) rating
But how do we apply certain talents and special abilities:
Double Damage per hit or when using Blast (X) weapons just add 1d5 after all other hits have been resolved
Storm of Iron:
Doubles Magnitude Damage (When do we apply this?)
Metal Storm Bolt Shells:
Bolt Gun gains the blast (2) special quality
So how do these 3 powers work in concert?
Morgan, a Storm Warden Devastator Marine fires his Heavy Bolter at a horde of Tau Fire Warriors, he scores a tremedous 10 hits, to make sure he cuts the horde down to size he uses Metal Storm Bolt Shells
2d10+8, Pen 4, Tearing Blast (2)
Since he is using a (X) based weapon he receive a bonus hit so he hits 11 times (we assume he wounds).
He is using a weapon with Blast (2) so that is 2 damage per hit makes 22 Magnitude damage
Now all hits have been resolved and we add +1d5 magnitude damage for Unrelenting Devastation (we rolled a 9, so +5)
Now Morgan causes 27 Magnitude Damage....but we are not finished yet...
Morgan is a ery experienced Storm Warden and has recently bought the Storm of Iron Talent which doubles Magnitude Damage
27 x 2 = 54 Magnitude Damage in Total...