Welcome to the sometimes frustrating world of airguns.

As to barrel variability - yes this can occur, but generally such variability is not to the extreme you are experiencing.

You mention you have tried cleaning the barrel - how thorough was the cleaning?

Sometimes lead deposits can be stubborn to shift.

You also mentioned at some point that the rifle has been stored in a case, is there a possibility that some moisture could have been trapped in the barrel that has caused a patch of corrosion in the bore?

One other thought, IIRC the HW100 has a two piece action block - is this still firmly bolted together as if the scope is on two piece mounts it could theoretically cause some movement if the two halves are loose.