It is stated that Shepherd was dishonorably discharged from the military (before it was expunged). However, as an officer, he cannot be dishonorably discharged. The equivalent term for a commissioned officer is a "dismissal".
A dishonorable discharge from the military disqualifies an individual from police/detective service. Dr. McKay reveals the fact that Detective Sheppard was dishonorably discharged after a botched attempt at rescuing a medic from behind enemy lines in Afghanistan. However, he also stated that the record was expunged for political reasons.
Near the end, when the A-10 Thunderbolts make a strafing run, you hear the one plane firing what sounds like a heavy machine gun. The A-10 does not have a machine gun: it has a Gatling cannon (GAU-8/A Avenger) which, when fired sounds like a short buzz, because it fires supersonic rounds at 4,200 rounds per minute.