This guide shows some recommendations and possibilities to instantly kill an animal using bolt-action rifles. Play the game and see what works best for you, that's not necessarily the same as described. It's purely based on in-game efficiency and not on ethics! In the beginning it will be difficult to perform precise shots as you have few callers and don't have experience, perks and a good scope. You may want to wait with spine, neck, heart and brain shots until you have those, meanwhile aim for the lungs.

Important tips for getting those instant kills:

• For each species use a weapon that gives you 100% integrity bonus. There's an integrity table for all rifles in a section below. The best rifle for each species is also in the description.
  
• Use the zeroing perk, you can choose between 75, 150 and 300 meter for bolt action rifles. Default is 150 meter. If the zeroing isn't close to the actual distance it will be difficult to aim precisely.
  
• Get close to an animal before shooting, it makes aiming easier and you will do more damage as the penetration is higher. 75 meter is ideal with zeroing perk, further than 180 meter is not recommended for highly accurate shots. When aiming at the lungs the distance makes a significant difference in damage!
  
• Level up the rifle perks to decrease aim wobble and other useful perks.
  
• Polymer bullets have better penetration, so more chance to hit several vitals.
  
• Wait for an animal to stand still, it's way easier to aim precisely. Take your time! Use the best rifle scope you have.
  
• Use callers to lure animals close. They will stop every now and then, that's the moment to take your shot. Most callers are effective if the animal is within 200 meter.
  
• Make sure the wind is not blowing towards the animal. Otherwise use scent eliminator but that will not remove it completely!

Chinese version (translation by Lin Li)
Other guides: Tips, tricks and animal difficulty/rating - Animal location maps

• Best option for small game (e.g. jackrabbit, coyote, fox, lynx, roe deer, springbok). Ideal if the animal is broadside, head-on is fine as well but it can be difficult to hit more than one vital for deer.
  
• For medium game it's only recommended if the animal is broadside and within 150m/164yd. You'll usually need to hit both lungs or a lung, liver and stomach for 100% quick kill bonus. This will be difficult if you're using weapons with low power (e.g. .243). If a good angle at the neck or spine, that can be the better option for an instant kill.
  
• For big game (Gemsbok, Moose, Bison, Buffalo) it's not the most efficient option, as it will often cost a lot of quick kill bonus. Even hitting both lungs won't give you 100% quick kill bonus. It will kill the animal though, only aim for the lungs if the animal is broadside.

• UPDATE: since a recent game update thoracic spine shots are not an instant kill anymore!
  
• Works very well for all medium and big animals, if broadside. Also sometimes a good option if the animal is resting, or standing up (bear) while facing away.
  
• Avoid the scapula (shoulder blade), it's very hard to penetrate and will likely prevent the bullet from reaching the spine. That makes it tricky to hit the thoracic spine if the animal is quartering-toward (facing you at an angle).
  
• If you aim too low of the thoracic spine you will still hit the lungs. That's not the case with the lumbar spine which is also smaller target.

• Very efficient for deer (Roe, Fallow, Red, Whitetail, Blacktail, Elk, Moose, ...) if head-on or rear-end (facing you or away). Get close for high accuracy.
  
• Aim right in the middle of the upper/middle/lower neck.
  
• If head-on be aware of the head, make sure you don't hit the jaws. If necessary shoot when the animal looks to the other side (left or right).
  
• If rear-end go for the upper neck, just below the head.
  
• If you don't hit the cervical spine (neck bones) it will be a flesh wound and it's not likely to die.

• Not always the best option as it's a small target and you need a good angle to reach it.
  
• You can hit the heart when the animal is head-on (lower chest) or broadside.
  
• It can be hard to reach for big animals.

• Only a good option for medium and big game if you can't go for the spine or the neck. That's the case for wild boars, bear, bison and buffalo if head-on.
  
• It's a small target and the head is very mobile so it's tricky. Aim between the ears, just above the eyes (and just below the horns if any).
  
• If you miss the brain and only hit the skull or jaws the animal is not likely to die.

This chart is true for one shot, subsequent shots will decrease the integrity depending on the class.

The only suitable rifle is the .22LR. Aim for the lungs, if you hit a lung or any other vital it is an instant kill.

Use the .223 for Foxes and the .243 for Jackal, Coyote and Lynx. Aim for the lungs, best if head-on or broadside. They are easy to kill as bullets have good penetration on them, hitting one lung should be enough for 100% quick kill bonus if close (within 200m/218yd).

Use the .223 for Musk Deer and the .243 for Roe Deer and Springbok. Aim for the lungs if broadside, hit a lung and another vital (other lung, liver, stomach) for 100% quick kill bonus if close. The .223 has bad penetration, so it can be difficult to hit multiple vitals, certainly at longer ranges (100m/109yd+). If head-on or rear-end (facing you or away), you can aim for neck (2nd). Another option if head-on is aim for the heart (3rd). Hitting the thoracic spine can be a good option in certain situations as well (4th).
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Use the .243 for Fallow Deer, Blacktail and Axis Deer, the .270 for Whitetail and Kudu (.243 works fine as well). If broadside the thoracic spine is a good target (1st and 2nd). Make sure not to aim at the scapula (shoulder bone), but just behind it. If the animal is head-on (3rd) or rear-end (4th) the neck is a good option. That will often be the case when using callers or if detected. If you go for the lungs you'll need to hit both and you may still lose some quick kill bonus. It's not recommended when using the .243, the .270 does have good enough penetration on Kudu within 150m/164yd.

Use the 7mm for Reindeer, Red Deer and Mule Deer, it does a lot of damage on them. When close (within 100m/109yd) hitting a lung and another vital (second lung, liver or stomach) should be enough for 100% quick kill bonus on Reindeer. Red Deer are a little tougher, you'll need to hit both lungs or one lung, liver and stomach. You have the best angle when the animal is broadside (1st) or facing you at an angle (2nd). When head-on you can aim at the heart (3rd). Other good options are the same as with other deer: neck bone if animal is head-on or facing away (4th) or thoracic spine if broadside (5th).

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Use the .338 for Roosevelt Elk and Moose (7mm is fine as well). Aim for the thoracic spine (avoiding shoulder bone) if the animal is broadside (1st and 2nd). Aim for the neck if head-on (3rd - middle/lower neck) or rear-end (4th - upper neck). If broadside and really close (within 100m/109yd) hitting both lungs or one lung, liver and stomach is also an option for Elk (5th), but not for Moose (won't give 100% quick kill).

Use the .270 for wild boar and warthog. If broadside go for a double lung shot, hitting a lung, liver and stomach will work as well. It may not be an instant kill, but still enough for 100% quick kill bonus if close (preferably within 100m/109yd). You can also try a thoracic spine shot for an instant kill (2nd). If head-on you can aim at the heart (3rd) or brains, just above the eyes in the middle (4th - hitting the skull is not enough!).

Use the 7mm for Black Bear, the .338 for Brown Bear. If broadside aim at the lungs, these weapons have very good penetration on bears. If you hit both lungs or one lung, liver and stomach you get an instant kill if close (preferably within 100m/109yd). You can also aim at the thoracic spine for an instant kill.

Use the 7mm for Puma and aim for the lungs. If you hit one lung and another vital (other lung or liver) you'll get 100% quick kill. When head-on aim for the heart, in the middle of the lower chest (2nd image).

Use the 7mm to kill Wildebeest. Aim for the thoracic spine for an instant kill. The animal needs to be broadside for a good angle, avoid the shoulder bone. If the animal is resting you can also shoot it from the back (2nd).

Use the .338 to kill Gemsbok. As with all large animals, hit the thoracic spine for an instant kill. Hitting both lungs is not enough, you'll also need to hit the liver to get 100% quick kill bonus (you'll need to be close to animal).

Use the .338 for European Bison, Cape Buffalo and Water Buffalo. You can also use the 470DB, but you'll need to be very close (within 80m/87yd, closer is better). These are very tough animals, aim at the thoracic spine for an instant kill if broadside. Make sure you don't hit the scapula (shoulder). If head-on you can go for a brain shot, in the middle above the eyes and below the horns. When eating or drinking, it's also possible to hit the thoracic spine from the front (last image).
Bison:
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Buffalo: