I have been a long time watcher of this forum and it has answered many questions for my gaming group. This is the first time I've actually found something that hasn't already been covered before so I thought I'd jump in and ask.
A Frag Missiles is Blast (5) Devasting (1): If it hits a horde it should do 11 magnitude damage.
5 hits due to the blast quality, each hit doing 2 magnitutde damage due to the devastating quality, plus the 1 for explosive damage that I do not think is increased by devastating.
A Cyclone Launcher is S/2/-: With the right roll I can hit a horde 3 times doing a total of 33 damage and, since I would be in tactical dreadnought armor and have the auto-stabilised trait, I could repeat that attack possibly doing 66 total damage in a single round.
Does Storm of Iron apply to the Cyclone Missile Launcher? Page 127 of the core book says when using a flamer or semi or full auto burst against an enemy with the horde trait you double any magnitude damage. It does not specify what type of weapon the semi or full auto burst is fired from.