Hellfire shift the Heavy Bolter to a single shot weapon with blast (3) and devastating (1).
So, you would get:
Heavy bolter--3 Hits
Explosive--+1, 4 Hits
Slaughter the Swarm: +5, 9 Hits
Devastating--+9 damage, (9 Hits, 9 Dmg)
Storm of Iron--x2 (18 Hits, 18 Damage)
Unrelenting Devastation: +5 (8 Hits, 23 Damage)
Tactical Experience: +1 dmg, (8 hits, 24 damage=32 Damage if everything hits)
Separately, since Slaughter the Swarm and explosive boost hits with the weapon, shouldn't they come into play before we double the damage dealt with Storm of Iron?
Your example is bit off. Most of it is correct, the following is just an overview for completeness.
Explosive damage effects are added after ALL other hits are calculated.
Slaughter the swarm applies hits after all other modifiers for hits are calculated.
These three are added all at the same time, after any special rules, traits, or abilities which increase or double hits.
Second, Devastating weapons deal the number in parenthesis in straight magnitude damage for every hit dealt. So this is applied after the number of hits have been calculated, but before damage is rolled on those hits.
Now that you've determined the number of hits you roll damage, hits dealing 1+ damage deal 1 magnitude damage to which you add the magnitude damage from devastating.
Now that normal magnitude damage is calculated you add:
Storm of Iron DOES NOT APPLY, since firing a hellfire shot is not a Semi or Full Auto Burst attack.
Finally, Tactical Experience and unrelenting devestation applies last as addative modifiers are applied after multiplicative modifiers and as they are special abilities tacking on extra damage they apply after the normal attack.
Essentially you have 2 phases: calculate hits and any traits or effects caused by hits, then calculate magnitude damage and any traits or effects caused by magnitude damage. Remember addative traits or abilities are applied AFTER multiplicative modifiers.
So your example should have been:
+1 for Explosive
+5 for Slaughter the Swarm (assuming your characteristic is 50+)
9 total hits
9 magnitude damage added to attack from Devastating trait
Roll damage for 9 hits (assuming all 9 deal 1+ damage)
18 magnitude damage to the horde from hits + Devestating trait
+1 bonus mag damage from tactical experience
+5 for Unrelenting Devestation (assuming you rolled a 9 or 10)
24 total magnitude damage. (13 without Slaughter or Tactical Experience)
The missile launcher is a little better than you give it credit for, first you've applied a talent to the hellfire shot you shouldn't and forgot talents which still apply to the missile shot (for fair comparison). Though there is not scatter on a miss for missile launchers in RPG, if you miss it goes sailing past forever or it hits a wall directly behind the target if your GM allows. Scatter is only for grenades, thrown, or other special cases.
+5 from Slaughter the swarm (assuming characteristic 50+)
11 Hits (assume all do 1+ damage)
+11 Magnitude Damage from Devastating
+1d5 mag damage from Unrelenting Devastation (assuming 5)
+1 mag damage from Tactical Experience
28 total magnitude damage (17 without Slaughter of Tactical Experience)
Your example of a heavy bolter weilded by a dev with several anti-horde abilities and using specialized anti-horde ammo is obviously going to outperform anything else against hordes. Against heavier targets, especially with the -2 damage/pen of the metal storm ammo, this guy is going to have a harder time. If the horde takes cover making several hits fail to damage with the weaker ammo the amount of dealth decreases.
12 normal hits (perfect shot: 6 blast(2) hits)
**remember multiplicative modifiers are added BEFORE addative**
(though I'd personally house rule this away, the rounds blow up several feet before hitting the target)
+5 slaughter the swarm (assuming characteristic 50+)
18 normal hits roll damage (assuming 1+ damage but with damage and pen reduced by 2 some probably fail to hurt)
36 magnatude damage from Storm of Iron
+1d5 Mag damage from Unrelenting Devastation (assumint a roll of 9 or 10)
+1 for Tactical Experience
42 total Magnitude damage (31 without Slaughter or Tactical Experience)
So the damage spread isn't quite that bad, but with specific tools tailored for the job it gets deadlier. Even with those tools a smart GM can still limit the pain (using cover or tougher enemies in the horde for instance). All the horde damage from metal storm rounds come mainly from the large number of hits. If those hits do not deal damage to the horde they don't do magnitude damage and therefore the do not get multiplied by the other talents. One hit failing to do damage in the last example woul drop mag damage by 2, if half fail to do any damage the final total is 23 magnitude damage.