A head shot, with 3*more criticals, finesse, 10 Luck, better criticals ...
Shooting at an Endurance 1 target should have a knockout chance of ~39.27% per shot, knockdown is additional chance of ~47.2%
Shooting at an Endurance 6 target should have a knockout chance of ~35.5% per shot, knockdown is additional chance of ~28.32%
But then again, on a more general char
A head shot with just Luck 6, with better crits ...
Shooting at an Endurance 1 target should have a knockout chance of ~26.29% per shot, knockdown is additional chance of ~32.36%
Shooting at an Endurance 6 target should have a knockout chance of ~24.33% per shot, knockdown is additional chance of ~19.42%
Not too far wrong, or at least not an autolose to fight against