But Republicans said the purchases raise "serious" questions about waste and accountability.
He noted that is "roughly 1,000 rounds more per person."
"Their officers use what seems to be an exorbitant amount of ammunition," he said.
Nick Nayak, chief procurement officer for the Department of Homeland Security, did not challenge Chaffetz's numbers.
However, Nayak sought to counter what he described as several misconceptions about the bullet buys.
Despite reports that the department was trying to buy up to 1.6 billion rounds over five years, he said that is not true. He later clarified that the number is closer to 750 million.
He said the department, on average, buys roughly 100 million rounds per year.
He also said claims that the department is stockpiling ammo are "simply not true." Further, he countered claims that the purchases are helping create broader ammunition shortages in the U.S.
The department has long said it needs the bullets for agents in training and on duty, and buys in bulk to save money.
While Democrats likened concerns about the purchases to conspiracy theories, Republicans raised concern about the sheer cost of the ammunition.
"This is not about conspiracy theories, this is about good government," Rep. Jim Jordan, R-Ohio, said.
Rep. Darrell Issa, R-Calif., who chairs the full Oversight and Government Reform Committee, said he suspects rounds are being stockpiled, and then either "disposed of," passed to non-federal agencies, or shot "indiscriminately."
If that is the case, he said, "then shame on you."