It does makes sense to have backups. He could have learned the lesson from the Colorado movie theater shooting and the clackamas town center mall shooting. The larger than standard capacity magazines tend to cause malfunctions.Bumpstocks don't change the ejection pattern unless the rifle gets away on you. But do we have ALL the pictures of the spent casings somewhere or are people just going off of one picture? Oh btw. They said several of the rifles jammed. I suggested that he may have had all them to account for malfunctions, overheating etc and some on here said no one would ever use more than a rifle or two lol. If I were plan something like this I would want as many loaded guns ready to go as possible. It just makes sense.
1100 pieces of brass will not even fill medium flat rate box to the top. We are only talking about a 50 caliber ammo can worth of brass here. That is not going to be plies of brass especially when scattered throughout a couple of rooms.
I highly doubt he got off 1100 rounds, the videos of the shooting did not appear show the acoustic evidence to support a large round count. Of course I have not seen all the video footage there may have been more than 3-4 steady 11 second bursts. At 600 rounds per minute he would have had to fire 11 steady 10-15 second bursts to get that many rounds off.