At the age of 20, unless military (which he was not), Lanza wouldn't have a lot of hands-on with firearms.
Yes, I was in the military.
Somehow I doubt the “tactical reload” and FPS shooter explanation. They fit the narrative too well, although they may be true.
My money is on Lanza’s mental state as the explanation for the mag changes and his shooting behaviors, such as shooting one child 11 times.