I worked all aspects of security/law enforcement during my short 18 years as an Air Force Security Specialist. It is in no way inconceivable for anyone to bring anything onto an AFB, or even into a restricted area.
The closer you got to a restricted or priority area, the more intense security was. Very few areas had security metal detectors.
A medical facility, with the exception of the pharmacy (which is a controlled area), was not a restricted area, nor was it even listed in the controlled area realm.
Security is expensive and it can not always be handled with mechanical means. Being that people are involved, there will always be some degree of fallibility in any security operation.
It is my understanding that this was a medical center for deploying personnel. As such, I am surprised there was even that much security at the building (The two civilian LE personnel outside).
When we join the military, we give up, or are forced to curtail, certain rights to protect those rights that we feel are important to us. I had the freedom of speech, but not on the base and not in Uniform. I had the freedom of the second amendment, but not while on base, nor while I was under the control of the base. It is ironic, but true.
I wonder if the major (who, in my not so humble opinion, is a traitor not only to this country, but also to the men and women he served and those who served him!) had to go through the process of getting the provost Marshall's approval to purchase the firearm? This is going to be a very interesting case as it unfolds, but unfortunately, being the military still insists upon political correctness, instead of personal accountability, I do not think we are ever going to hear the full report.
Inside Every Bright Idea Is The 50% Probability Of A Disaster Waiting To Happen.