Found this yesterday at the range yesterday; out about 15 yards in front of the firing line, like someone purposely threw it there. I've never seen or even heard of a firing pin breaking off AND sticking in the primer. As you can see, the pin is rusted pretty good, so it may have been laying there for awhile. After I took the pic, I noticed that the 6.5 Grendel case has been necked to 22ARC. Luckily for whoever the unlucky soul was, an AR15 firing pin is pretty easy to replace.

OUCH!

Do you think that crescent on the case head is from the ejector? It looks too big for that…

Maybe you can see better with it in hand than I can see in the photo but with the single image, I’m not convinced that is a firing pin nub. It appears more like a primer cup that flowed in to the hole in the bolt face. And it looks like a pin hole and leakage at 7 o’clock.

Definitely an ejector mark, and a harsh one at that. Somebody crafted something poorly, we will agree on that.

Sevens, you are correct. I deprimed the case and this is what it looks like from the other side. "Somebody crafted something poorly, we will agree on that"; well said.
What a fired primer is suppose to look like and the primer in question:

Yes, protrusion looks like the anvil inside every primer.

I certainly wonder what the whole story was here but we will never know.

One thing we do know, over pressure

I cleaned our range for many years. I would occasionally find gun parts. In my den I still have a piece of a stock from a 98 mauser that I recovered on the ground. It had been blown off from some gun malfunction.