The shooter had a 20 round clip in his rifle and used it all. When arrested, it was empty
Possible, but unlikely. Most SKS have a 10 round fixed magazine, fed with 10 round stripper clips. There are aftermarket 20 round fixed magazines, and varients that use AK-47 removable magazines. He could easily have emptied it, reloaded from a stripper clip, and emptied it again. Articles only say that about 20 rounds were fired and that the SKS was empty when found.