Sadly what you see are all indeed spent. The only kind of thing i have got from all this is to see that one magazine is still full on the floor.
The only thing stopping him doing worse will be the barrel overheating as these guns are only designed for short bursts of full auto. He will have had to change over to the other weapon then and repeat that. People will have had a few vital seconds to run. This imo is why he had so many and speaks to his determination to do absolute maximum damage, disturbing stuff really. He knew he only had a short time frame to do what he did and given the weapons he has available to legally purchase could probably not have done more damage if he tried.
The more i think of it the more it seems clear this is almost textbook max damage one man could inflict in a major USA target, his positioning, the lack of law enforcement. The modifications he had were the max he could get under US law (i think) that within reason he could smuggle in. Really, i don't think he could have done more damage as a non military trained person,he was ruthlessly efficient and prepared. There is no "thank heavens it was not..." or "at least they did not do..." here and it makes it very hard to take. This was cold, calculated and thought out to the best a person probably could.