Originally Posted by
DarkenedHue
Even without Missile Barrage, AM is the best use of your mana at 4 stacks. Its mana cost isn't increased by the Arcane Blast debuff. Again, this is assuming you care about being mana efficient (if you weren't, then you'd simply continue AB spam until MB procced).
As for using Arcane Barrage instead to force a reset, well, you will be queuing AM immediately after the 4th Arcane Blast anyway in the hope that the 4th AB procs Missile Barrage. It's going to be less efficient to wait until the cast has ended and only then hitting Arcane Barrage after you see Missile Barrage hasn't procced.