Understood. That is why the comment that these large clips are generally not purchased by the general public caught my attention. Ordinary clips are not traceable, but a monster clip may spark a memory for the seller.
There is no reporting on sales of magazines, you could buy from anyone in the world ... literally. It’s a fools errant to try and trace magazines, or limit the capacity.
Reloading is not that difficult, if practiced. This person knew what they were doing, and likely practiced, simple as that. Move, shoot, reload, it’s a drill practiced if you are serious about defense. It is not that hard a drill to master or become proficient at.