I've seen another way those cracks are made BR.  On the 92/96 I have that is free floated, even the receiver ring is free floated.  Since there's a little space there, getting on the front guard screw with my honking big screwdriver that I use spread out the area behind the magazine resulting in the cracks.  Got those stabilized, and my receiver ring floating like someone planned it a long time ago. Rear screw first, really tight, front screw a bit less tight, on this one.