I'm looking for a little help on how to figure out how to calculate damage to Hordes when using the following setup:

Suppose I have an arm-mounted pistol (let's say a bolt pistol) mounted on one arm and a basic weapon wielded in the other hand (let's say a bolter). I'm an ultramarine with Storm of Iron and Slaughter the Swarm, and I want to just open up with everything I've got against a Horde of bad guys. How do I go about calculating hits/magnitude damage for this?

Storm of Iron says that it "doubles the amount of damage done to a Horde's magnitude. For example, if the Battle-brother's attacks would reduce the Horde's magnitude by 5, this amount is doubled to 10." Slaughter the Swarm says "when calculating the number of hits inflicted against a Horde, the Battle-brother scores a number of bonus hits (after all other modifiers) equal to either his intelligence bonus or his perception bonus (whichever is higher).

So if I open fire with my bolt pistol and bolter, each on semi auto, and assuming I roll max hits, and have +4 int/per bonus, would it go like:

Bolt Pistol (2) + Bolter (3) = 5 (x2 Storm of Iron = 10) + Slaughter the Swarm (4) + Explosive (1) [or 2 for using two explosive weapons?] = 15 [or 16] Mag damage?

Multiply first, add last.

First for hits, then for magnitude damage.

Storm of Iron (and Whirlwind of Death) is a special case, it requires magnitude damage to be calculated normally (as above), then doubled.

So your example:

5 hits total + 4 StS bonus + 2 from X (1 from each weapon) = 11 hits total

Roll damage on all hits (if necessary), all hits deal 1+ damage do 1 magnitude damage.

Assuming all do damage:

11 magnitude damage

Now double magnitude damage from SoI:

11 x 2 = 22

Your example should have dones 22 magnitude damage based on the aformentioned assumptions.

I thought explosive did an additional point of horde magnitude damage for each point of horde mag damage. So in the given example it should look like this.

5 hits total + 5hits for explosive + 4 from StS bonus = 14 hits total. Now double for Storm of Iron = 28 magnitude damage. Just how I figured it.

If I am wrong please site sources, not that I don't trust you personally, I just like to read it for myself (and GM).

The errata, page 6, under damaging a horde: "Weapons that inflict Explosive Damage (X) gain a bonus against Hordes, and count as having inflicted one additional Hit per Attack after all other Hits have been applied.”

The one thing I think you're shorting yourself on is Slaughter the Swarm, I see nothing that says it only applies once per turn, and when attacking with the multiple attacks action, they are treated as two, distinct attacks. Both can hit, both can miss, both can be dodged, and both have modifiers such as explosive apply, so I can't figure out any valid reason why both wouldn't benefit from slaughter the swarm. Unless I'm missing an errata or clarification from somewhere, the way I'm seeing it is this:

Attack 1 (Bolt Pistol): (Max) 2 hits, + 4 (StS) + 1 (Explosive) = 7. Assuming all deal at least 1 point of damage, that causes a total of 7 points of magnitude damage. x2 for Storm of Iron = 14

Attack 2 (Bolter): (Max) 3 hits, + 4 (StS) + 1 (Explosive) = 8. Assuming all deal at least 1 point of damage, that causes a total of 8 points of magnitude damage. x2 for Storm of Iron = 16

Total: 30 points of Magnitude damage.

How about if we throw bolter drill and hellfire rounds into this? And a stormbolter instead of a standard bolter?

Pistol => 3 + 4 (StS) + 3 (HfR) + 1 X = 11 x2 (SoI) = 22

Storm Bolter => 5 + 5 (Storm) + 4 (StS) + 5 (HfR) + 1 X = 20 x2 (SoI) = 40

62 Mag damage?

No, not quite. Hellfire rounds deal one additional magnitude damage per hit, they don't add to hits.

Pistol: 3 hits (max, with 4+ dos) + 4 (StS in the current example) + 1 (X) = 8 hits. Assuming each does at least 1 damage, that's 8 magnitude damage + 8 (hellfire rounds) = 16 magnitude damage, X 2 (Storm of Iron) = 32

Storm Bolter: 10 (max, with 8+ dos) + 4 (StS) + 1 (X) = 15 hits. Assuming each does at least 1 damage, that's 15 magnitude damage + 15 (hellfire rounds) = 30 magnitude damage, X 2 (Storm of Iron) = 60

Total: 92 magnitude damage

Note that for you to be dealing at least 1 damage with each hit, it must be a horde of unarmoured cultists or something like that (and 'wasting' specialty ammo on them while you do so), and 8+ dos is only at best a 50% chance (and that's with your BS being absolutely maxed), so in reality it's not quite as impressive or dominating as it looks on paper. Still, any time you can fire 13 bullets and kill potentially 7x that number, it's not too shabby.

Is there any rule against dualwielding storm bolters? Or did you just fulfill the starting example?

Is there any rule against dualwielding storm bolters? Or did you just fulfill the starting example?

You need two weapon fighting which from my searches is only available to tactical and assault marines without a GM advance.

Errata makes it available to all at rank 7 I believe under general space marine advances. I dont have my computer in front of me at the moment.

Errata makes it available to all at rank 7 I believe under general space marine advances. I dont have my computer in front of me at the moment.

and here is the errata quote: