All of the examples you gave in your link were done with semi automatic rifles or handguns with the exception of 3. According to your link, out of the 133 incidents listed as mass shootings, only 3 of them were done with shotguns. That is around 2 percent that were committed with shotguns. And no, if shotguns had been used for all those other mass shootings, the injury/death rate would have in all probability been much less, as when the shooter stopped to reload, the police would have had a much better chance to stop them. Why else do you think that more than one weapon was used, as well as the fact they mainly used semi automatics in those incidents? They didn't want to have to stop to reload and wanted to throw as much ammo downrange as possible in the shortest amount of time.