South Carolina regulations call for the shooters to fire bullets “[…] to immediately stop – the heart”.
But the autopsy report commissioned by the SCDC indicates there were only two gunshot wounds, not three, and that the bullets hit his pancreas, liver and lower lung, and largely missed his heart.

Revealed: Autopsy suggests South Carolina botched firing squad execution
Records obtained by the Guardian indicate shooters missed the target on Mikal Mahdi, which lawyers say caused prolonged suffering