Not sure about ethical killing range but the .223 kills them (Wallaby) just fine at 425 yards. What would make it unethical for me would be shooting at that sort of range without good gear, a good rest and little wind.
Ive shot deer with the .223 out to 375 yards often enough but with 69-77 grn hand loads (mostly) stoked along and under good conditions.
Don't wish this age on yourself. Make the most of your "now".
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