if you look at the crack, its been cracked for awhile, it just finally let go.
+1....gotta agree with TAB.
I've seen that a thousand times in the industrial maintenance field. Mostly with hydraulic cylinder shafts, but the characteristics are the same.
Look at the two dissimilar colors of the metal in the photo. The light color is the fresh metal that just gave away. The darker has been "open" for a while. The dark discoloration is most likely from absorption/coating of solvents and lubricants. From looking at the size of the crack, it is a testament to how strong these rifles really are.

In this photo the area has been wet with something, but you can still see the difference.
