Yeah, no idea on harder knock. I'm not sure I would allow that as a GM if it was me, as a gel round is a gel round is a gel round. No matter how much you aim, you're going to hit the armored glass instead of an exposed part of a person.
And, this is all assuming the Stuffer Shack has armored glass for windows, which honestly I wouldn't find all that surprising. Probably cheaper in the long run.
ETA:
Oh, and if you were to resist the DV using the Shooting Through Barriers rules, which I personally think is much more reasonable than the Penetrating Weapons rule, the Ballistic Glass would resist with Structure 8 + Armor 12 for a total of 20 dice, statistically averaging 6.67 hits and reducing your base DV from 13 to 6.33, or less than the structure rating of 8. This would mean that your attack would fail unless you got 3 or more hits, at least from a probability standpoint. DV12 base +3 vs 20 dice resist = 15 - 6.67 = 8.33 DV, or enough to overcome structure rating. This would result in 7 DV being transferred to the target, which, again, I find to be a much more reasonable rule than the penetrating weapons malarkey.