To study the forming regularity of combustion residue caused by the deviation of web size of gun propellant, the web size of oblate propellant was measured by a spiral micrometer and the distribution rule of web size was obtained by statistical analysis. Using propellant form function of composite charge, the distribution rule of web size was converted to propellant form function, and then the relationship between distribution of web size and combustion residue was investigated based on the classical interior ballistic models. Results show that the deviation of web size exists in gun propellant, and the distribution rule of web size conforms to normal probability function. The propellant with large web size can not be burned out in chamber and form combustion residue. The deviation value of web size of oblate propellant is 0.10 mm, and 0.2% of the propellant charge will become combustion residue when firing. When the deviation value of web size over 0.02 mm or expected value greater than or equal to 0.29 mm, part of the propellant charge will change to combustion residue, and the amounts of residue increases with the increasing of expected value or deviation of web size of propellant.