The firing chamber is the larger section of the Spud Gun where the buring of the fuel takes place. It is usually a section of four inch PVC or ABS plastic pipe in schedule 40 thickness. This pipe is cut to the desired length and glued to the four to three inch reducer at the front and the female four inch clean out end cap. The rough volume of this chamber is important so take that into account when cutting the length of four inch pipe. This rough volume can be found from V=p r²

Remember also to take into account the volume of the reducer fitting and end cap if precision is required. The best way to determine the exact volume of the chamber is to fill it with water and measure the volume of the water contained in the camber with graduated measuring devices such as beakers. This volume is important to know so that the proper proportions of fuel to air can be added for correct combustion.