I commend you for admitting your error. We are human, and no one is perfect; Murphy does not discriminate. Like others who have read and comprehended this thread, we are all thankful you were OK, learned a valuable lesson, and shared your lived experience with us. You are a better person than most in doing so.Thanks to all for the responses. I do own 300 black although not shooting it that day. I am going to assume that somehow a 300 BO round got dropped in with the 556 ammo previously. My bad for missing it when I loaded the mag. I am lucky and humbled by the experience. Ashamed to say that I assumed a 300 blackout would not chamber in 556. Definitely a learning experience. Attention to detail is the key.