My dad had a similar story. When he got recalled they had to shoot a bunch of nearly depleted ammo for 155 mm arty.
They sit in the field waiting for the command to fire. Guy that has the pull cord had been smoking some weed and was sitting behind the gun drumming.
One of his beats is with the hand in which he has the cord, it was slightly to strong. The gun fires and he gets the recoiling breech block in his face.
My dad jumps on him and keeps his tongue from sliding back in the guys throat. Shouts at the commanding officer to get a medic, finally the guy gets wheeled out by an ambulance. Nothing heard from the guy.
Years later, in a bar he hears some drumming he recognizes. It's the same guy, he got small fracture at the base of his skull and got an army pension for life. Buys the beers for my dad the rest of the evening.