From Smith and Wesson:

1) With a loaded magazine in the firearm and the bolt locked to the rear, depressing the bolt release to allow the bolt to drop freely may ignite the round as the bolt closes without engaging the trigger and with the safety selector in either the safe or the fire position, and may also result in multiple discharges.

2) With a loaded magazine in the firearm, bolt in the closed position and a round in the chamber and the safety selector in the fire position, depressing the trigger will cause the round to fire normally, however as the bolt cycles, the next round may be ignited by the bolt crushing the rim of the case as it closes, causing multiple discharges.