Actually, the interior walls aren't that hard (as the grenade showed)
Average Material with 4 Structure and 6 armor
Since you use a penetrating weapon p.198 core would apply:
PENETRATION WEAPONS If the weapon you’re using is primarily a penetrating weapon, like a firearm or a pointed sword, then the barrier takes 1 box of unresisted damage (or no damage at all at the gamemaster’s discretion), allowing the rest to transfer to the target behind it. When multiple rounds are fired at a barrier, the damage increases to 2 boxes for 3 bullets, 3 boxes for six bullets, and 4 boxes for 10 bullets. Subtract this from the damage done to anyone on the other side of the barrier. This is only true for weapons whose modified DV exceeds the Armor rating of the barrier. As above, if the modified DV is less than the Armor, the attack is stopped dead with no damage to anything.
