Received a call from my son today regarding something I have never encountered or even heard of. While practicing with my 58 cal Lyman Trade Rifle using 90 grains of Pyrodex RS and after 3 successful shots, he misfired. He recapped but misfired again. After waiting an appropriate time, he cleaned the nipple and added a few (very very little) grains of Pyrodex RS in the nipple and shook it down so it would ignite properly. Upon firing the rifle, the shot was successful, but it also blew the nipple out of the drum and just past his head. Scared the Hell out of him. There doesn't appear to be any thread or drum damage, but he couldn't recover the nipple. The nipple was a standard 6x .75mm as required by Lyman rifles. My question is 2 part. Anyone else ever experience such an event? What was the cause if known? I have been shooting every type of BP firearm on the market and have NEVER had this happen. I have theories but no data to go by. Any one???