Sad to hear. I constantly preach safety to the group that hunts at our farm. The little things can sometimes be what gets you in trouble.
Sounds like an obstruction caused the problem. ML are often preloaded since they are not considered loaded until the primer is in place. (in Ohio anyway). It is easy to forget that you did that, then a second bullet is loaded...boom.
Hope he recovers.
Keep life simple...if you hear hoofbeats, think horses, not zebras.