I have shot rats with both .22 and .177 to good effect and had good results in both calibres, .177 is my prefered choice for shooting whatever quarry and aslong has you hit the rat in the neck/head it will be brown bread,they are pretty tough critters and i have seen them run on 20yrds before they drop stone dead even with a well placed shot,obviously with head shots this does not happen but they still can jump about a little till the nerves switch of,but sometimes a head shot is not possible,so in my opinion from my experiences either calibre is more than capable of stopping a rat if shot acc