I assumed AM was already inside the feed room, called Paul to come to him, and shot him when Paul entered the room, causing the ejected casings to stay within the room. I don't think think the first shot completely incapacitated Paul, and he was shot again while he was already trying to exit the feed room. That's why Paul is laying half in/half out of the room, head pointed toward the exit.
I don't fully understand how Paul's brain matter was shot upwards toward the ceiling, unless he was shot from slightly below?
After AM shot Paul in the feed room, AM continued to wait in approximately the same location until Maggie got close enough and he tried to shoot her, but she didn't go down right away and he had to move closer as he was shooting.
all IMHO