Now first of all, I am going to tell you the downsides. Getting the Ballistic Skill 10 is situational. You need the right type of target which is not normally a vehicle. Here are the negatives
*it is a psychic test at leadership 8 to use this power.
* to get BS10, the shot must be fired at a unit of psykers or a unit that has a psyker in it.
*perils will kill your shooting unit dead.
*Its an assault 1 36" S10 AP1 large blast shot. This allows you to shoot it from a moving vehicle just fine.
*At BS 10 you are almost guarenteed a hit, or at worst you just might scatter 2"
*With 36" range and being assault 1, you can move to keep your distance from any psychic hoods.
Here is how you do it.
Take an Ordo Hereticus Inquisitor for your Grey Knight army HQ. The rarely used item that is required is a psyocculum. A psyocculum does the following if you have not already guessed.
The bearer of a psyocculum (and his unit) count as being Ballistic Skill 10 if shooting at a psyker, or a unit containing one or more psykers.
So that being said, its simple.
Your inquisitorial Henchmen are the following....Psykers (8). A psychic barrage is S10 AP1 with 8 psykers in it. I generally add in some other units as well, just incase of perils so that the unit is not a complete loss or loss of a victory point.
There you go. Want to go hunt some psykers? Here is one weapon in your arsenal that you should consider taking along with you. This is also fun when you take an orbital bombardment with a techmarine and join him to the unit.