I think the Short Barrel, Carbine Length Gas System and Shooting Suppressed probably were the reason for a crack like that showing up.

Those three together, that's just about as hard as you can run an AR... unless you add F/A into the mix.

Good to know that a Mil-Spec carrier will run to around the 7,500 round mark that they're rated for even in a SBR.

I wonder what kind of condition the carrier key is in, mind taking photos of the carrier too, I bet it has some unique wear in other places...

Were you running 5.56 mostly or a mix of .223 and 5.56?