Have you ever thought that 7.62x39mm and .243 Winchester can be a pair of cartridges that are dangerous if mixed? Of course, any cartridge is dangerous if not used in a gun it is chambered for and nobody should do it. However, there are some pairs of cartridges known for causing catastrophic failures if mixed.
When we talk about dangerous combinations, what immediately comes to mind is .300 Blackout shot in a .223 Rem chamber. That is perhaps the most significant flaw of the .300 Blackout design. But cartridges like 7.62x39mm and .243 Winchester look so different – they have different dimensions of cartridge head, case length and it is strange how could someone mix these two and how could they even chamber and be shot in each other’s chambers?
It turns out, that ball FMJ bullet of 7.62x39mm cartridge will get stuck in the .243 Winchester chamber just at a right spot where its primer would be positioned in the bolt face so that the action will close and the striker will be able to hit the primer. It kind of headspaces on the bullet. I assume that spot is where the neck portion in .243 Win chamber starts.
Again, nobody should mix cartridges, but accidents happen. Particularly to the gentleman in the video below. He chambered (not intentionally) a 7,62x39mm cartridge in a .243 Win rifle and shot it. The result was a blown up rifle because he was trying to push a .311 caliber bullet through a .243 bore.