300 Blackout round loaded in your chamber is the most likely cause. The pictures of your upper, the magazine, and chamber are nearly identical to 3 other incidents of 300 Blackout being loaded in a 556 or 223 Wylde chamber that I have personally witnessed as the former range master for a major police department on the west coast. The 30 caliber bullet hits the lands as the actions cycles and is pushed backwards into the case causing a compressed load. Coupled with the bullet resting against the lands which most reloaders know can cause a pressure spike, this is the recipe for the disaster you experienced. The bolt rides forward enough to go into battery and when the trigger is pressed KABOOM. 2 rifles had regular loads and were both destroyed causing minor injuries to the shooters. The 3rd rifle fired a sub sonic round. In that case the bullet came out of the barrel like a 4 inch long piece of lead spaghetti, which I recovered just behind the target stand. This rifle basically stayed together, but upon close inspection of the chamber it was found to be cracked.
JMHO