Over-Penetration is the Sniper's nightmare. It means that the bullet passes clean through it's target without tumbling, thus causing far less damage than would normally be the case. This rule is designed to simulate this problem and force Sniper characters to choose their ammunition with care rather than simply loading up on HEAP every time.
Over-Penetration occurs when the PEN of a round exceeds the HP of the area hit after passing through any armour the target is wearing. A round that over-penatrates will still damage the target. The target takes half normal damage (round down) for the round. AD is taken as normal.
This is not the only problem with over-penetrating rounds. Should the Sniper be in a situation where it is important not to damage the suroundings or people other than the target an over-penetrating round could be a serious issue. To see if the round ends up somewhere it shouldn't have the sniper make a luck check (2D10 +/- good or bad luck, 11+ to succeed). If this check is failed halve the DAM, PEN and AD of the round and apply it to something that shouldn't get hit. If the roll is double 1's then apply it to the worst possable thing to get hit.
EG: Toe Rag the ganger is threatening Sonic's cub. Sonic takes aim with his Fen Tri and puts an AP round into Toe Rag's left lung. Unfortunatly Toe Rag is far less tough than he makes out (10 HP) and is only wearing a bike jacket (be generous, PV 1). The PEN of the round is 12 (12.7mm AP) so it has overpenetrated. Toe Rag takes 4 damage to his torso. Fortunatly for Sonic the luck check comes up as a 15.