Its unfortunate that over the years we have tinkered with so many different guns, and that our memory gets distorted

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AirArms started the 400 series with a two piece brass transfer port that was lock tightened to the barrel.Later they made it a one piece. With the two pice you needed to take the barrel out and seperate the brass pieces to replace the probe o ring. The one piece made it better as you didnt need to strip anything,
Things to note:-
The barrel needs to be airtight once air is charged.
The two piece or one piece transfer port needs to be lock tightened and sealed (and a new oring sandwiched in between the brass pieces if a two piece) A gap will distort the pellets skirt when passed over it.
The transfer port needs to be aligned fully.
If the old barrel has been taken out as the original op has had, then tell tale signs could help its location. this could be witness marks left by silencers or even using a magnifier to find fresh bluing that has been protected by the air cylinder.
I have also never had to rotate and fire a new barrel to find the perfect poi

Hence my open question. I have rolled one on a flat surface to see if its straight

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Please understand that members on here are giving advice free and as they have experienced it. All answers have been given in good faith and have been given to help not hinder.
Well done Eboswan for some excellent moderating.
BBF