Many things could have gone wrong with this.
1) Brandon Lee situation. Squib type load. Projectile in the barrel. Then blanks used which pushes the head down the barrel.
2) Plugged barrel weld or like internal devise became loose and was propelled like above. This differs from a military BFA. Many gas operated weapons need plugged barrels to operate (AR15). If not, you have to pull the charging handle after every round.
3) Live ammo used (unlikely, but possible). Mix up of prop ammo and live ammo. If it was a revolver, many movies, you see the head in the cylinder when watching from the front.
We will have to wait to see.
For 2 people being shot, it could have been a frontal scene so he is shooting at the camera. That's if it was for a scene or rehearsal. Then maybe went thru 1 person and into the other. Unknown yet. If this was not a rehearsal, like they're just goofing off or something, then that's a different story.
Some are saying that the actor should know the condition of the gun and check, or don't point the gun at anyone anyways. That's not how Hollywood works. Actors rely on the prop guy. And the actors focus on acting, not props. Especially if the gun isn't a main part of the storyline like John Wick. But even then, does Keanu load his own mags with blanks to verify, or does the prop people do it for him and hand said gun to the actors.