He was firing on troops doing PT. They probably weren't carrying weapons.
When I was in BCT (B-3-1 Ft Ord, Oct 1966 since everyone seems to be bragging on this thread) We occasionally did "Rifle PT". But then, I suppose it's kind of pointless with the lightweight weapons they train with today. Also, in those days, our M-14s were kept in an unlocked rack in the center aisle of the barracks - not "Dorms", Barracks; spit shined center aisle, foot lockers, butt cans, fire guard at night. Just like in the WWII John Wayne movies. They don't do that anymore.
My Dad used to tell a story about two rival platoons in adjacent barracks that got into a gun battle shooting at each other from the barracks windows. That was at one of those temporary training camps in the South in about 1942 or 43. My cousin, who was in the Marines in the early 1960s actually brought his M-14 to our house in the trunk of his car when he came to visit.
Anyway, by the mid 1970s the Army, at least, had locked up the weapons in an Arms Room and only issued them out for training or drill.
This guy was shooting with a personal firearm, a pistol, not his issue weapon. Lucky for the guys doing PT I'd say.