Overpenetration/key hole refers to shields
I.E.: Omnigul has an arc shield and golden gun is solar. Instead of 1/4 of the GG damage being used to destroy her shield
4/4 of it goes straight through (keyhole) and damages her health
English
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That's not how that works. Over penetration refers to pushing through multiple enemies. It used to be like that. Even without over penetration, golden gun can do damage after shields are down, and it doesn't pierce shields before they fall.