Patricia is shown firing the shotgun three times and never operating the slide between rounds. She would not be able to fire more than one round without reloading it via the slide.
The wound on Rainer does not line up with the hole in the door- it is the wrong height and much larger on him than it was the door.
Every time Patricia fires the shotgun, there was no recoil. It was apparent she was not firing a gun and the sound effects were added later.
Angelo was shown hunting birds with the shotgun, which uses birdshot- very small pellets. When Patricia fires it through the door and again at James, the resulting holes would not have been possible.