Yeah, I am going to go with "Smells like bullshiat"
Three on the left are 300 blackout, #4 is 5.56.
In case the img link doesn't work:
File:Five bullets.jpg - Wikipedia, the free encyclopedia
Unless, and this is a big unless, the neck tension was inadequate in the 300 round and it slamming into the chamber pushed the bullet farther into the case (and the powder charge was such that there was room in the case to do so). It is also possible that the rifle was not completely in battery.