Honest question, and I'm not trying to be like MOP- I just don't know the answer, but: didn't they change security tactics at Fort Hood and other military bases after the last attack? In order to prevent this very sort of thing from happening again?
Did they change the security measures on the West side of Chicago where there are ten shootings a day?
If you live on Fort Hood you can have a gun in your domicile, as long as you register it with the base. If you don't live on base you could probably sneak one in if you were wanting to confront someone (which seems to be the case here). This is a base with 40k troops, you can't search every car.
The reason this story or any story about a shooting on a military base, is they are pretty infrequent as compared to the general population. Also considering what happened at Fort Hood a few years back, it becomes a monster story.