Since DHS swallowed up 22 fed agencies and includes among others the US Coast Guard, Border patrol, and Secret Service, I would expect them to buy a lot of ammo. They also run FLETCE, the "Police Academy" for all the Fed agencies, and they also train state and local cops as well. Look at some of what they are buying:
.45 Long Colt
.44 magnum and special
12, 16, and 28 guage shotshells in birdshot, not buck or slugs.
That doesn't sound like they are buying up ammo preparing for an armed conflict, that sounds like a whole lot of agencies have used DHS's buying power to get deals on oddball stuff they need. I seriously doubt they plan on arming DHS agents with Winchesters and Colt SAAs.....
In any event, even if ya armed every single Fed employee and sent them out to do battle, they would be outgunned 30-1 at least.