Rabbinical students were given an exemption because of how the holocaust had decimated the Jewish religious establishment. Their numbers have recovered and the exemption is no longer necessary.
an AR-15 with a now-legal bump stock can fire off 900 rounds in a minute.
The rate of fire is 900 rounds/minute, but the shooter has to stop every 20 or 30 rounds to reload. Effective rate of fire would be maybe 60 rounds/minute.
Figured that out all by yourself, did ya? ;-)