A while back I had a chance to fire an AR with a "bump stock" at my local range and took a good look at the set-up. At that time, I told the owner that, as far as I was concerned, it was not legal and should not be legal. It would be an interesting legal battle, but an attachment that makes a semi- into a full-auto is thereby illegal because a full-auto is illegal without a permit. If one modified the action to full-auto, the weapon would be illegal. I one modifies the stock/trigger to fire full-auto, it is illegal.
I am quite interested in finding out, if we ever do, how this lone gunman was able to amass several dozen AR-type firearms, modify them with bump stocks, smuggle them into a Las Vegas strip hotel which has impervious security, stayed with them for several days in a room which he modified, altered the hallway by installing cameras, had food delivered by room service, brought in several thousand rounds of ammo, likely practiced and sighted-in rifles in the weeks before, and not a single person seems to have said "huh?". No member of his family, none of his friends, none of the local law enforcement saw anything suspicious. Reports say he fired 1,200 to 1,700 rounds, which is a considerable pile of brass. Yet the pics released by the FBI of the room show only a dozen or so. The bump stock only works when firmly bedded into the shoulder and a certain amount of forward pressure on the front pistol grip: a posture very difficult to maintain while shooting out a hotel window at targets 400 yards away and several hundred feet below.
Doubtful we will ever know the shooter's reason for wanting to kill large numbers of indiscriminate people assembled in outdoor events. Apparently he scoped out three or four other sites in the months leading up to this shooting and opted not to act.