Kjohn,
my guess is that it broke after ejecting the case and when slamming down into the receiver.
The broken off part looks more worn on the 'upper part' on the picture, this may indicate that it has been hitting the receiver unevenly. Perhaps due to a missmatch between the location of the hole for the rear toggle link pin and the height of the rear toggle link. Just a thought..
Was it a matching gun? Is there a dent in the receiver?
Some other theories would be that the rear roggle link pin was too small, headspace wrong, too large bolt punched in between rear and middle toggle link, +P ammo.. It could be a lot of things. The important thing is that you came out of it ok
