headshot from back or front , spine at the base of the neck - from behind, or chest - you have more range than you think since at anything under 5m you are essentially firing single ball. irrespective of what is in the chamber. You need to be careful with a headshot since the brain is high up in the skull - easy to go too low and blow the jaw away.
If you go for the chest then go just behind the front leg and just under halfway up to blow the heart out - if the animal is down then make allowance accordingly.
The thing with any firearm is to make sure that the line of fire is clear - although shotgun is better in this regard than rifle
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Excellent guidance, thank you!