Episode 33 of "JAG"
s02e11 Force Recon

Harm goes undercover as a Marine gunnery sergeant to help Mac in her investigation of questionable Marine training methods and becomes a victim of unsafe maneuvers himself. While undercover, Harm learns that the tough-as-nails officer in charge of Force Recon training, Capt. Koonan, introduces unexpected problems into the training missions as a means of teaching the men to react quickly to surprises during combat. In addition to testing the Marines' mettle, the method also doubles their chances of returning safely from combat. However, in the mission in which Harm is involved, the method backfires dramatically, leaving five men trapped in a training maneuver destined for a disaster that even Capt. Koonan can't reverse, and it's up to Harm to get the trainees -- and himself -- out safely.