The performance of and the combustion in solid-propellant rocket motors may be affected by characteristic length \((L^*)\). For small characteristic lengths, the chamber pressure and characteristic velocity may be lower than in similar motors with larger \(L^*\). Oscillatory combustion may take place if the \(L^*\) is small enough. In the past, \(L^*\) oscillations have been explained by an oscillatory heat feedback mechanism and subsequent oscillatory propellant pyrolysis. The same phenomena - i.e., lower chamber pressure, lower characteristic velocity, and oscillatory combustion - can also occur in liquid-propellant, hybrid, and airbreathing rocket motors. It is shown that these phenomena may be explained by assuming a finite reaction rate in the gas phase in combination with small residence times. Even if a constant burning rate or propellant mass flow rate is assumed, the coupling between the energy release in the gas phase and the residence time is sufficient to cause oscillatory combustion. A stability boundary criterion for solid- propellant rocket motors is derived and is found to be in agreement with experimental results.