So, you're using 60 year old ammo, and it blew in the gun. That happens with 60 year old ammo. No surprise there.
If your gun was firing at 1,050 rpm, that's over 17 shots per second. So, about every 6/100ths of a second, a shot fires. It therefore takes less than 3/100ths of a second for the bolt to go each way, and maybe for less than 1/100th of a second, the bolt is locked and the firing pin strikes the primer.
If the primer delays by 1/100th of a second, the bolt is 1/3 of the way to the buffer by the time the cartridge fires. Could have been a bad primer, could have been bad powder with a slow ignition before the full charge ignited, could have been a casing that was internally corroded that couldn't take the strain of 50,000 pounds of pressure.
We all know that technically the MG42 can't fire when the bolt is open, but with 60 year old ammo, all bets are off.
Glad you weren't left bleeding in the video.... this could have been embarrassing. With a closed-bolt semi-auto, a 1/100th of a second hang fire would never have been noticed.
I'm not your mom, and I'm not slapping your fingers, but may I suggest that you shoot modern ammo in the future before you hurt yourself? The gun is capable of blowing with good ammo... don't decrease the odds of safety by using 60 year old ammo, which may not have been made properly, stored properly, or was just plain crap ammo to start with.
And is your anti-bounce spring in the bolt carrier facing the right direction? Some of them are directional.