I have read similar things. I know nothing about guns so I can't make any judgement. The sequence of shot sounds in some of the personal videos that have been posted on youtube do not line up, if you know how gunfire sounds, with how the sequences would be using those attachments the shooter was supposedly using to make regular guns fire like automatics. Some of the sequences are similar to what an AK-somenumber automatic firearm would sound like. Because of all the tall buildings any recording would include not just the sound from actual shots but the sound of the echos those shots made, but even that does not explain the differences. One personal video I remember being specifically mentioned was one by a taxi cab driver.
I believe there is way more to this than we have heard and will maybe ever hear.