I cannot make out any "cracks" in you picture, so I cannot tell whether what you have is common, or not.
However, mine does not have anything that I would call "cracks," so it is probably not common, nor normal, whatever you are seeing. * ;D
If you keep slamming the bolt on mis-ejected cartridges, I would not be surprised to see damage occur, though, just my 2c. * :
Anyway, I have seen some FTE on my 9mm Sub2K, as well. *It has only
happened to me on the last round of a full magazine. *The first time out, no problems at all. *Maybe 60-75 rounds when I bought it.
Second time, I experienced the FTE on the last round of the full magazine, happened both with the stock 10-round Glock magazine, as well as with a 33-round (Glock fctory) mag. Always the final round.
Gave it a thorough cleaning, occurred a little less frequently, and did not
happen if I did not fill the magazine to full capacity (i.e.
5 rounds in a 10 round mag = no issue).
I am wondering if this is result of not getting the action area cleaned sufficiently (but only after 150+/- rounds?), or a flaw in the machine? *:P
I might add, this was all with cheap (UMC and Federal cheap) ammo. *Will consider higher grade, but at 60 rounds (or more) per minute, that could get expensive! *:'(