You are faced with a grave situation. Your horse is suffering and either poses a severe and immediate hazard to itself or handlers or it is in severe pain and distress with no hope for recovery.
It is always best for your vet to evaluate your horse, confirm that euthanasia is the most humane (or only reasonable) choice, and to perform the euthanasia themselves. But this is not always possible.
When performed properly, a gunshot induces instantaneous unconsciousness and does not require close contact with the horse.
Ensure that other people and animals are far out of the line of fire and not near the horse when it falls. Be aware of the potential for ricochet.
See the accompanying images for the landmarks. Draw an imaginary "X" joining the outside corner of each eye with the outer base of the opposite ear. Point the gun just above the intersection of these lines. Aim the firearm directly down the neck, perpendicular to the front of the skull, and held at least 6 inches away from the point of impact, and fire.
Immediately after you fire the gun, the horse should collapse and may experience a period of muscle contraction or spasm that usually lasts no longer than 20 seconds. This is often followed by a period of relaxation and kicking or paddling movements. The pupils of the horse’s eyes should become dilated (large).
Check your horse within 5 minutes to confirm death. There should be no breathing, no heartbeat or pulse. If you touch the clear surface of the horse's eye (cornea), there should be no blinking or other response. If there is any response or blinking, you may need to shoot the horse again.
Tips for safety & Success
Do not euthanize your horse unless you are completely confident in your decision and ability.
The proper location of gunshot penetration is critical to the destruction of the brain and to minimize suffering.
Do not simply shoot a horse between the eyes! As you can see from the image, this is far too low. A .22-caliber long rifle is usually sufficient to euthanize a horse, but a 9mm or .38-caliber handgun will be more reliable, as will large caliber rifles.
The use of hollow-point or soft nose bullets will increase brain destruction and reduce the chance of ricochet. If a shotgun is the only available firearm, the use of a rifled slug is preferred.
Euthanasia with a gun should only be attempted by individuals trained in the use of firearms. Care must be taken to minimize the danger to the operator, observers, and other animals.
You must comply with all laws and regulations governing the possession and discharge of firearms. Local ordinances may prohibit the discharge of firearms in certain areas.
Consider where you will do this, because you will also have to properly dispose of the body afterwards. Weigh this against your desire not to cause your horse undue pain, should it be difficult or dangerous to move them.
Helpful Outside ResourcesCredible Equine Health Information on the Internet