Republican Rep. Jason Chaffetz said Thursday that the Department of Homeland Security is using roughly 1,000 rounds of ammunition more per person than the U.S. Army, as he and other lawmakers sharply questioned DHS officials on their "massive" bullet buys.

"It is entirely ... inexplicable why the Department of Homeland Security needs so much ammunition," Chaffetz, R-Utah, said at a hearing.

The hearing itself was unusual, as questions about the department's ammunition purchases until recently had bubbled largely under the radar -- on blogs and in the occasional news article. But as the Department of Homeland Security found itself publicly defending the purchases, lawmakers gradually showed more interest in the issue.

Democratic Rep. John Tierney, D-Mass., at the opening of the hearing, ridiculed the concerns as "conspiracy theories" which have "no place" in the committee room.

But Republicans said the purchases raise "serious" questions about waste and accountability.

Chaffetz, who chairs one of the House oversight subcommittees holding the hearing Thursday, revealed that the department currently has more than 260 million rounds in stock. He said the department bought more than 103 million rounds in 2012 and used 116 million that same year -- among roughly 70,000 agents.

Comparing that with the small-arms purchases procured by the U.S. Army, he said the DHS is churning through between 1,300 and 1,600 rounds per officer, while the U.S. Army goes through roughly 350 rounds per soldier.

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."