Alright, here's the skinny: (I think)

This was really bothering me, so I started looking for some justification here. I spent about an hour searching the net until I found this. The link says that the SSA has an IG office with about 460 criminal investigators and auditors. IF these guys carry guns, the 174k rounds would afford them approximately 375 rounds per agent. No confirmation on whether or not they are armed, but if they are this sounds like a fairly routine and legitimate purchase.