Sheridan Blue Streak is 5 mm/.20 cal.
Those pack a good punch! Years back I had a Beeman Kodiak in .25 cal - what a beast it was to break open to cock it (~40 lbs of force). Spring air rifles are lots of fun.
Blind him w/ a flash light and use a net to tangle him up. Walking him outside and place in a pail of water. LOL or you could be like Bugger and stomp em w/ ya boots!