I have this same issue. I have a gully that I can shoot down into with .22LR subs but that is only about third of the shots I need to make on bunnies.
Have HW95 .177, while a lovely springer, I don't shot it very well and pretty limited in range. Just putting a pellet into the lungs, so not a quick kill (bare in mind it's straight out pest control), and very unsatisfactory.
Long term a .22 cal PCP is the answer but that is pretty big $$.
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