Hokay, sorry about that: I meant to put more but I got busy. I'm assuming you mean Missile Barrage rather than Arcane Barrage (ie the free Arcane Missiles that you get when it procs, rather than the 51 point spell). There's much debate as to which is correct, but essentially the rule seems to be that you build up 4 stacks of Arcane Blast, then use the proc of MBarr. If you don't get a proc of ABarr then you have 2 choices - either cast Arcane Barrage to reset the stacks of Arcane Blast and start fishing for the proc again (most mana efficient) or simply keep chaincasting Arcane Blast until you get a proc, then consume your MBarr proc with AM and start casting AB again (most damage).
The general rule of thumb seems to be that you keep casting AB until you get the MBarr proc and then eat the proc with Arcane Missiles UNLESS you're finding that you're struggling for mana. Given that the art (as far as I understand) of the Arcane Mage is to dump your mana as quickly and for as much damage as possible, then fill up and start again I'd say that this is your optimum stance EXCEPT in gimmick fights where mana management comes into play (such as Vezax in Ulduar).
