Try this, load one round at a time by hand and shoot single shot style, ease your bolt down and fire. That way your checking ejection and any possible firing pin problems. I still think your firing pin stuck for it to fire when the round was only 1/2 way seated in the chamber. Pin may have a burr on it from the factory or a slight bend which binds it in the tunnel causing it to stick. For your safety and others, DO have it checked.