Just so we’re clear, “DHS” stands for Department of Homeland Security. It’s a bit of a misnomer, because it doesn’t really protect our homeland (such as, from foreign invasion or conquest), it is more like a Federal police force. That is, it’s purview is only those living in the United States. Thus, anything it buys is in furtherance of its mission of being a Federal police force within U.S. borders. Thus, if it buys 1.6 billion rounds of ammo, DHS is arming itself to for possible conflict(s) with people here in America.
And whom does DHS view as threats? Military veterans, people who support the Constitution, people who want a return to the rule of law, Christians, home schoolers, etc.
Oddly, DHS is worried about people who own guns and stockpile ammunition, but it isn’t worried about itself.