Here is a photo showing how .300blk can be chambered in a .223 chamber. We have seen many examples of the aftermath of this scenario once the shooter pulls the trigger. This cut away shows how there is enough room for the cartridge to be chambered. Just like safe firearms discipline one should pay attention to what ammo is going into the gun. Perhaps this image may help explain it to you or someone you know.
POTD: Informative .300 BLK Cutaway