ShinSung Shinsung Fire Leak

I'm fairly new to PCPs, and don't know much about their workings.
I have a Shinsung Fire 201 that has a leak that I cannot figure out how to stop. I have checked the o-rings, I have blasted with air to try to seat the o-rings, I have tried filling with it upside down, and I've tried dry firing while filling
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it upside down. It's a big leak. Any help is appreciated. Photos attached showing location of leak. (The spot in the circle is just a nick in the metal from removing that pain in the butt end piece.)