Max's Glock locks open, indicating the magazine is empty, after firing only four rounds. A Glock holds more rounds than that, a typical Glock 9x19 mm holds 15-17 rounds, a .40 S&W holds 15 and a .45 Auto holds 13. There are smaller capacity magazines as well, some states do not allow magazine capacities over 10 rounds. However there is no Glock magazine that only holds four rounds, the smallest capacity magazine they make holds six rounds, and that is only for the subcompact models, Max's Glock is a full sized model.
In the hostage situation as depicted, NCIS would have no standing in the operation, especially since two of the hostages were NCIS employees (this would compromise any decision making). NCIS jurisdiction is limited to offenses involving USN and USMC personnel.