At the very beginning of the clip the rifle is out of sight so we can't tell what's happening down there.
No, that's not correct.... The rifle's metal butt plate is clearly visible. Dan, can you do a freeze frame at the beginning of the film?
Day he is already crouching down with his hands on or near the rifle.
Yes, that's true....But, I don't believe he has touched the rifle at the very beginning of the film.
Dan, I don't believe I've ever seen a better copy of Alyea's film.... That metal butt plate on the rifle is very obviously HORIZONTAL.... Look under Lt Day's left elbow just as he starts to lift the rifle.
This Gif shows the first few frames from the moment Day appears on the Alyea film slowed down.
His hands are down where the rifle is.
The rifle is out of view.
The Alyea film does not tell us the rifle was lying down when Day picked it up.
I believe, after moving some boxes about, Day reached over the rifle and grabbed the strap.
The second he pulled the strap the rifle flipped to the horizontal position.
To a certain extent I imagine the scene has been staged but only in terms of telling Alyea to come and film the moment the rifle is picked up - 3..2..1..Action!

The photos are real.
The rifle is found in an upright position.
Somehow the clip is still inside.
Alyea films the moment the rifle is picked up.