They said they were firing at quite a rate, it could equally be some spastic in charge of an off ticket illegal .22 rimfire. Doesn't sound any worse than an air rifle and will fire as quick as you can pull the trigger if its a semi such as a ruger 10/22. Report it and don't say its an air rifle as you don't know for sure, the police are more likely to do something if it could possibily be live rounds. As face it, it could have been.