Random note… In assault spec, full auto does almost exactly as much damage as two charged bolts, for only slightly less ammo (more up front, so it's pretty neutral) and nearly identical proc chance on high impact bolt. I'm not sure why you would advocate full auto under those circumstances, especially since you can get the proc on HiB on the first CB and won't need to wait out the cast.
Full Auto, in fact, has a 75% chance to proc HiB. That is 30% more than Charged Bolts. However lets say they were the same for a minute. I would still pick Full Auto because each of the 3 ticks of Full Auto has a chance to apply Plasma Cell. Those same ticks also get 30% increased damage if they crit, whereas Charged Bolts would not.
Also keep in mind that Full Auto will proc HiB at the beginning of the cast while Charged Bolts procs at the end of the channel. During the channel of Full Auto gives me time to adjust if HiB in fact did not proc.
I hope this clarifies things.