I think that the main problem with the penetrating weapon rules is that "bulletproof" materials don't stop much in the way of bullets. Some examples:
Ballistic Glass (armor 6): Light pistols have a base DV starting at 6P, with a minimum of 1 net hit makes 7P DV, which penetrates the bulletproof glass and does 6P to the original target. This is, aside from hold out pistols, the lightest weapon available.
Hmnn.
I think things might not be as bad as all that. I was just re-reading the Barriers section.
Penetrating weapons is a modification of the Shooting Through Barriers rules. With Shooting Through Barriers, step 1 is, is the Weapon's DV higher then the modifed Armor of the Barrier? If yes, then
resist the damage by rolling Structure + Armor. Now, if you
don't have a Penetrating Weapon, you have to get past the Structural Rating of the Material. Otherwise the Barrier absorbs all the unresisted damage.
If you
do have a Penetrating Weapon, the Barrier only takes one box of the
unresisted damage, or 0 if your GM is feeling nasty. The rest go through.
So, against your 6P light pistol, yes, it can get through Ballistic Glass. Let's say you get 4 hits, dead average on 12 dice. So that's 10P. The glass is resisting with 10 dice, so that's probably 3 hits, for 7P. Subtract 1 box, and you're looking at 6P again. Not quite as bad as what you were postulating, where the defender is looking at 6P from only one success on the roll.
Of course, with glass, you still get your defense test. So in that case if you hit, the glass is effectively giving you another plus 10 armor.
Kevlar wallboard would be better. In that case, the barrier is resisting with 20 dice. So, your 12 DV gets resisted with an average of 6 successes and gets knocked down to 6DV, with an AP of -1. Still not great, but more survivable then getting hit directly.