The characterization of the combustion properties of the aluminum-magnesium fuel-rich propellant for unchoked ducted rocket was studied. The pressure exponents calculated by the burning rates at different pressure from the pressure gauge and from the absolute pressure are different. The author suggested using the exponents calculated from the absolute pressure. The experimental results show that the correlation between the burning rate and the pressure is better suited by the Summerfield formula than the Vieille Law.