@colinjallen Thank you for your answer. From the time is wrote the post above and reading your answer, I stripped the gun again. Looking down the barrel and the hop fully open or closed, bb's just fall through. After adding a piece of foam to the hop (just as an experiment), I finally could see a lip forming on the bucking. Reassembled the gun (at this point I could not get the fire selector in place and it's full auto all the time), and did a test fire. My hobby room is 4 sqm, and I didn't even hit the wall....
Tomorrow, I'll run strip it again and follow your instructions. When you say feed tube, you do mean the opening of the hop unit, correct? In regards to the hop unit, I have attached a picture of it.