Two things you got wrong Breeder was you forgot to apply the AP of the blast to your damage soak... so the barrier has armor 12, -3AP for 9... then roll x2 armor for the soak. Physical barrier spells are special... if they're breached at all the spell falls apart.
According to the S4A I have, page 167, yer suppose to ignore the AP of the weapon when attacking barriers to destroy them. And I really don't see a grenade being able to selectively attack
through a barrier.
Secondly, don't double the damage each time it bounces... it's the same damage each time the wave laps against the shore (barrier). So it's 2x6 the first time, 2x6 the second, 2x6 the third if it still stands... and an unlucky character takes 6+6+6==18S. (probably unconscious barring drugs, pain editor or the like).
Hang on, let me write this out. Blast travels from the grenade, and crosses the target the first time. That DV 6S. It bounces off the barrier, crosses the target a second time. Now its 12S. This time, when it hits barrier, it breaks through, leaving it at 12S. Now, the blast wave that started out going the other way (away from the target, bouncing off the barrier, and back again) does the same thing, increasing the DV to 24S. Do that for the other four sides (six total directions; forward, back, left right, up, down), and you get (6x12=) 72S.
Hrm. Okay, looking at it another way....
Wait, that would only matter if the blast bounced three or more times before breaking through. But that didn't happen here (the barrier breaks instead of rebounding after its DV is increased to 12S). So, there is that.
And wouldn't the wall effectively receive the same damage as the target, in the case of the stun grenade? After all, the rebound effect has to hurt the walls too.