What Happens When You Shoot 300BLK in a 5.56 Upper

I’ve made the mistake of shooting the incorrect cartridge in a firearm before, it was a .40S&W in a 1911 chambered for .45ACP. I lucked out and there was no damage. Unfortunately for a Redditor on r/AR15 his gun wasn’t so lucky. Here’s a link to the full-size...