I create a Force Dome at PR 10, extending 5 meters all around me. If someone 10 meters away shoots at me, I have an additional 20 AP.

But if I shoot him back, does he also get 20 additional AP? Or does the Force Dome only protects those "inside" of it?

Force Dome only protects characters inside of the dome. This includes friend and foe.

The vagaraties of the warp care not about physics. Shots going through the dome, out the other end, and hitting someone or thing behind it hit with full force.