And to add to the number of people talking about how important the arrow is to this equation....make sure you have flawless arrow flight! One of best ways to do that, is to build your arrows without fletching and shoot them over and over. Yup, that's bare shaft tuning at it's best. If you can get your arrows cut to the length that the spine perfectly matches your bow at your drawlength, the fletching will impart spin and simply tighten up your groups.
Now your arrows will fly as straight as possible, not wagging back and forth like an over excited golden retriever's tail. If you arrow is wagging side to side, or porpoising up and down, and strikes the target at the extreme of the wag, lots of energy and penetration is lost. Or worse yet, the arrow it turned from a rapidly killing shot to one where the animal suffers for days before dying.
Arrow placement, sharpness of the broadhead, and arrow flight cannot be over empasized.