Social Security Admin. orders 174,000 rounds of ammo
And its answer, that it must supply and train 295 special agents, raises questions of its own.
I think many agencies view having their own law enforcement, not to mention a SWAT team, as matter of prestige. A JP in New Mexico once issued a number of search warrants for food stamp fraud cases. Next morning he found the town overrun with Dept of Agriculture SWAT teams, taking down people that he knew would have turned themselves in if given a phone call. Overhead was a helicopter full of armed guys. But there was nowhere to land it ... too many telephone lines. He asked an agent and was told it would just hover there for a while and then fly back to the airport. It was only there because Congress had started asking why the agency had a helo and never used it.
UPDATE: I'd agree that there are thousands of federal buildings that need some entrance security, and that those folks ought to be adequately training. But in my experience, Federal buildings are not guarded by employees of the agency involved, if only because (apart from major HQ buildings) most federal buildings combine several agencies. They are guarded by Federal Protective Service, which is part of DHS. This might explain DHS's large purchases, but not that of other departments.
I can remember when Interior was protected by private agencies, with unarmed employees, and no metal detectors. When there was an alert they looked in your briefcase, and ladies' purses, but never under overcoats. I thought Interior did not want to face the grim fact that no terrorist would think it worth attacking. What are they going to knock out, the environmental law division of the Park Service, or the migratory bird hunting division of Fish and Wildlife, or screw up FOIA response by taking out the Division of General Law?