This type of question comes up all the time. It's a good question. The most important element of killing an animal humanely is bullet placement and bullet construction. The .243 is perfectly adequate for deer if these two factors are present. To illustrate: I was a taxidermist for 25 years, and during that time the largest bull elk that ever I mounted was killed by a 16 year old with a .243 using Barnes bullets. Although I wouldn't recommend this caliber for elk, this bull was shot in the heart and dropped in its tracks. I've seen deer shot through the lungs with a .243 that dropped just as fast. The heavier calibers give a little "cushion" if the shot happens to be a little off for whatever reason, but they really don't kill any faster or any "deader" under the right conditions. The .243 may actually be a better choice for a young person or someone who is not used to heavier recoil as this relatively light load will not result in the shooter developing a flinch. Good luck!