years ago i was reloading on my 550 and right at down stroke there is a huge bang. same thing, check fingers, check drawers, deaf, etc. turns out it was my double garage door spring deciding to break. great timing.
had this happen on the last stage of a major...single shot AR. couldn't figure it out on the 4 hr drive home. next day went to clean the gun, split it in half and happen to look into the lower and staring me in the face was a spent primer that had lodged itself under the rear of the trigger. bizarre.