EE was using a WOMA gun to spray build up in the riser duct, 3rd fl, Tower. He was in a squatted position spraying throw the lowest access door on the W wall's S corner next to the 5th stage S meal pipe. A chunk fell onto the wand of the gun causing the gun to throw ee striking his R arm against the meal pipe and mouth by the stock end of the gun as he fell to the floor.