Regards to horde damage, a frag missile does 2D10 X Blast (5) Devastating (1)
So, it is blast which means and I quote "A Blast weapon that hits a Horde hits a number of times equal to its Blast value" So, Blast (5) means it hits 5 times. Devastating does "If a target is a Horde, it reduces its magnitude by a number equal to the number in parenthesis every time it is hit by this weapon." Now, since it only has Devastating(1) it does not really do anything sine 5*1=5. After everything is said and done, X type damage causes 1 addition hit. So thats 6 damage to a horde and +1d5 if you are a Dev
Then we move on to Metal Storm rounds. A Dev uses a Heavy bolter (HB). He scores 5 hits. Metal Storm rounds have the Blast (2) quality so every hit in fact causes 2 hits to a horde. That doubles the damage to a horde up to 10. Add in the X damage and you are up to 11. Add in that 1d5 for being a Dev.
Does that look about right?
Metal Storm looks right. 10 hits (5 Blast (2)) +1 hit (Explosive) +1d5 damage (Devastator) = 11 + 1d5 damage (as long as all hits damage the target)
Frag is incorrect (as far as I am aware). It would be Blast hitting 5 times. Devastating does 1 Magnitude Damage for every hit so that would be 5 (I used to think it was once per attack, but the errata changed/clarified it, and made some weapons ridiculously good against hordes). Explosive inflicts 1 extra hit (I am not sure if this sets Devastating off again). Roll damage 6 times (5 times for Blast, and once for explosive effect). However many successfully damage the target (frag missiles are one of the weapons most likely to fail to do this) are added to the total damage. Add together,
5 hits (Blast) + 5 damage (Devastating) + 1 hit (Explosive) + 1d5 damage (Devastator) = 11 + 1d5 damage (as long as all hits inflict damage, which is less likely in this case).
The Missile launcher may get another +1 damage, if the hit from Explosive activates the damage from Devastating.
The two look fairly comparable, though there are some differences. I think Devastating does its bonus damage per hit regardless if the hit damages or not, so a successful missle hit does a minimum of 5 damage (+1d5 if Devastator), but as the damage is very swingy (a flat 2d10) it is much more likely to fail to damage with its actual hits. The Heavy Bolter has lower minimum damage (theoretically just the +1d5 from the Devastator quality), and the number of hits is much more swingy (depending on degrees of success), but with the more reliable damage, most horde targets are going to be auto-damaged, meaning hits are automatic damage. The Heavy Bolter is also going to get the +20 from full auto, while the missile launcher won't.
Overall I would say Heavy Bolter is better for this task, but then it doesn't have the flexibility the Missile Launcher has.