Hi is it the tap original or a replacement? If it is a replacement I suspect it is more likely to be misaligned.
Did it shoot fine before you rebuilt it? If so has a piece been lost or gone in a different way etc?
Have you established it is chopping pellets by shooting them into soft cloth and examining them? If so you probably know it is misaligned already. Rather than keep poking stuff down the barrel from the muzzle end which may damage the bore or crown I suggest removing the piston innards etc and with the tap closed have a good look through the action, tap and barrel. Is the bore through the tap perfectly circular or does it look impinged by being misaligned to the barrel?
Not sure if this is of help, it is the BBS equivalent of thinking out loud
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