system
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human
ELI5: Why is Shooting a Deer in the Head a Bad Shot?

I'm not a hunter, but I'm dating someone from a family of avid hunters, and I wanted to know why a headshot is considered a "bad shot." 

I understand aiming for the lungs is a pretty much guaranteed kill, large area to aim for, etc. 

Modern power rifles would have no problem getting a kill when hitting a headshot, so guaranteeing lethality isn't an issue, so why is the head considered a bad shot?

Edit: It was pointed out that I should differentiate between taking a headshot and making the headshot. I appreciate everyone's responses about the head being an unreasonable target because it's small. What I want to know is making the headshot - say I'm a decent marksman and know I can make the headshot, why are the other hunters going to be upset with me for downing the deer? (And thank you to those that have provided answers to this question as well)
gpt
It is the same reason attempting to shoot a person in the head is a bad shot. (Assuming you are the general public, and not a navy seal).

Whenever you are trying to kill anything, be it human or animal, with a gun or bow and arrow. There are 4 ways to kill that target in what is considered a humane fashion.

1. Take away the targets ability to breath - puncture the lungs

2. Take away the targets ability to pump blood - puncture the heart

3. Take away the targets blood itself - puncture a main artery

4. Disable the central nervous system or CNS for short.- This is a head shot, breaking of the neck, or spinal cord up high above the shoulders. 

If you only have 1 shot to take. The central nervous system is the smallest target to hit. If you miss it, but still hit the animal, you can cripple it, but not kill it. (Look at gabby giffords, she lived through a head shot. She is all sorts of fucked up, but she is alive)

If you aim for the center of mass, that same one shot has the potential ability to hit the heart, lungs or an artery, or all 3 or any combination of the 3. Any 1 of the 3 will put down the animal in short order. And if actually happen to pull off a perfect broadside shot, you will hit the lungs, heart and an artery all in 1 shot. 

>Modern power rifles would have no problem getting a kill when hitting a headshot

That is simply not true. As a 17th century musket can do the same thing in the hands of a marksman. It always comes down to shot placement. Center of mass is a larger target than a brain. And technically speaking, you are not just trying to hit the brain, but a specific part of it. Hitting the medulla oblongata is the ultimate "lights out" sniper style shot that will disable the CNS as quick as you can snap your fingers. But with any head shot, If your shot placement is not dead on, all you will do is scalp a deer, but it will otherwise live. (or die slowly in pain and agony).  But if your aim is center of mass, you can still have a good hit which equals a clean kill even if your aim is off, or the target moves. 

Next is training. When you are first taught to shoot firearms (if you are lucky) you are taught techniques that are reliable, and more importantly, repeatable. Center of mass is a larger target than the CNS. At any kind of distance, you are much more likely to be able to repeatedly hit that center of mass simply because it is a larger target.  If you were trained to hit the smallest target possible, under stress, unless you are in the military and shooting tens of thousands of rounds a year, it simply isn't going to happen. Ones marksmanship would not be repeatable. 

And lastly there is what is tried, and what is proven. When taking game the whole idea is to cause the least amount of suffering to the animal. If you are a hunter, and your practices/techniques are shown to cause great pain or agony to an animal, then you are just a bad hunter. Over the course of the last few hundred years in regards to both firearm and hunting development. It has been proven that over the course of those hundreds of years that on average a center of mass shot is the most reliable and repeatable way to take game quickly and cleanly without causing any undue pain or agony to that animal. 

Last point. If an animal, like a deer, is dying a slow agonizing death. The body will secrete enzymes that do get into the meat, which can do anything from taint to destroy the flavor of the meat.