Thanks for all the advice, guys! Yesterday I took out the barrel and disassembled the hop up. Being a new gun, I assumed it's clean but the amount of dirt I dug out of the barrel is just unbelievable. The hop up wheel was very hard to turn so I thought I should oil it a bit too.
All went well but I had a big surprise when I looked through the barrel. With the hop dialed in my bucking was pressed in at the left edge of the barrel window, which is left of the axis. As I twist the barrel I can see the pressure point moving from left to centre to right and back. I tried to centre it and even move it to the extreme right but each time I took the barrel out after a couple of shots, the nubbing was pressing on the left again. Could this make the bbs fly to the left by giving them a left spin? They actually go left anyway even with no hop.
As I said, I checked the alignment of the sights by sighting through the barrel. However, with the rear sight centred I hit 10 cm to the left, with the rear sight all the way to the right I still hit 3 cm to the left (from 8 m). The frustrating part is that my groups are really tight with touching holes. I am planning to get a new hop up and bucking maybe a barrel too but not right now. Or maybe a rear sight with a bit more adjustment?
Happy shooting, everybody!