I have used .177 .25 also the humble .22 they all did the job but personally I have found that .22 suits my needs better I found that the .177 could cause more passing threw quarry casing one or two runners which needed a follow up shot also needed more accuracy to hit a vital spot the .25 has great stopping power but a fair amount more drop off when opening up the ranges so I now stick to the .22 as I say it suits my needs better and gives more error forgiveness at sub 12ft lbs