Accuracy
PRACTICE. Not only will improving your accuracy increase your likelihood of a killing shot, but it is also critical to safety.
Your firearm may demonstrate different accuracy with different ammunition. One bullet and powder charge may hit the bullseye, but a lighter load may miss the target completely.
It is important to practice with your game rounds as well as any back-up or secondary loads and your practice rounds to ensure not only your accuracy, but also your equipment's accuracy with varying bullets and powder charges.