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 image.
From the Reddit post:
Murphy’s Law struck hard this past weekend where some how some way a 300BLK round made it from the box on one side of the table, into the mag of another AR on the other table. No one was hurt due to proper PPE. The lower survived with just a broken bolt catch and the upper, as one would expect, is a complete loss.