I had pretty much the same thing happen some years back with a Thompson 1927 A-1. I'm still carrying a piece of brass in my arm from that little episode, and pulled another piece out of my chest where it hit the sternum and stopped. If that sliver had hit a bit lower, things could have been relatively ugly.
On that particular day, I was shooting at steel plates, had pulled the trigger and............nothing. Hmmm. Waited a few seconds keeping the barrel aimed downrange, then looked up and saw the bolt hadn't closed. A jam.
Keeping the barrel downrange, I waited a little while, then pulled the bolt back to clear the jam. Something we've all done. You keep the barrel downrange, finger off the trigger, wait a bit to make sure you don't have a hang-fire, and clear it. No big deal, right?
Well, as I pulled back on the bolt, the firing pin let go, hit the primer, and the round went off in the open chamber. I remember seeing a blinding blue-white flash. Looked over at a couple of buddies, and we're all thinking what the.....?
Then one of the guys says "Uh, you're bleeding. Are you okay?"
Looking at my chest, I see a little piece of brass sticking out. I pulled it out, and discover it's about 1/4" long. My sternum had stopped it. Then I notice a little blood on my right arm from a small pucture. X-rays the next day revealed another brass splinter up against the bone.
I've thought back on that incident more than once, and other than turning the gun upside down so the receiver would be facing down before operating the bolt to clear the chamber, I'm not sure what else I could have done.
I still have the case and the piece I pulled out of my chest. The case is completely flowered, and missing a few pieces. The bullet ended up about an inch up the tube. I keep them in a little plastic case in my desk as a reminder.
I count myself lucky.