Placement, angle, and close contact with the bolt stunner is key. I mentally draw an x between eyes and ears, and angle straight down and slightly back, as if I were aiming for the rabbits throat area on the other side. I press pretty firmly, which you would think would upset them, but they actually go still. I have them in a little shoe box sized box for the adults, and a skinnier box for the fryers, they back up against the butt bumper and just freeze.
please excuse the less pretty diagram,
@TroubleMakerAcres did a better job. But the placement and angle are just the same for gun or bolt.
Most often if it goes wrong you have angled left or right of center, or forward, then you miss the brain for an eye socket or nasal cavity. I do spend some time kind of rubbing a little circle on their forehead, feeling for the spot that is centered. Probably up to 20-30 seconds at least. The rabbit tends to press it's chin to the floor, making finding that perfect angle easier. My ball shaped head is not accurate of course, the stunner is actually perpendicular to the rabbits forehead, which has a natural slope.
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