use whatever you find comortable and can hit the target every time, biggest problem will be finding the right pellet for the gun you choose, good start would be a AA s200 from my experience with AA field pellets, as for calibre the .22 has a lot of thump but for me i find a .177 easier to use over a longer spread of ranges as there is`nt as much of a curve on the pellet travel and at the end of the day a pellet through the brain will kill every time no matter the size