I'm intimately familiar with how domestic US military bases are secured. The reason we haven't had another 9/11 event is because nobody is trying to 9/11 us. The door is wide open when someone eventually does.
For your consideration...
-Depending on the size of the base, there's only a small number of armed military police on duty with an equally small back up force. About 1/4 of them aren't trained as military police, but temporarily pulled in from other military jobs to help bolster their presence.
-There's only enough rifles in the base armory to arm about 10% of the base, and that's if you're lucky.
-You're not allowed to have personal weapons on base with very uncommon exceptions. That means people living in the barracks are perpetually unarmed.
How do you think Nadal Hassan did the damage he did? Nobody had the capability to shoot back. This is the result of training people to be killers, then throwing them in an environment where they're treated like 3rd graders.