I think you could be onto to something. Agencies often purchase ammunition for more than one year, sometimes four or five.
How many agencies fall under DHS? How many employees are required to train and qualify with a firearm each year and how many rounds do they expend? I know a Federal Marshall who was complaining that his people were reduced to 1,000 rounds per year for training. 1,000 rounds per year multiplied by the number of sworn in his agency is a lot of ammunition and that's just one Federal agency.
MIL SPEC - Measure with a micrometer, Cut with an axe, Beat to fit, Paint to match