Every once in a while my gun wouldn't shoot a BB. It'll still shoot and cycle, but no BB comes out. As a result, 2 BBs are chambered. At first I thought it's either the barrel or hop up. Turns out it was caused by the nozzle internals, and I was using the SD POM set. I don't know what they are called, but it seems that the valve inside failed to return to the initial position, causing all the gas to blow back instead of propelling the BB first. I am now running the SD POM nozzle with stock internals, and it works.
