Days after the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives backed off a ban on ammunition that critics say could pierce the armor of police, Connecticut’s U.S. senators want residents to act.

"Americans ought to literally protest to the ATF and tell this agency that they've made a mistake by failing to stand up and be strong against this special interest industry,” said U.S. Sen. Richard Blumenthal, who joined New Haven police and U.S. Sen. Chris Murphy, a fellow Democrat, on Friday.

Amidst fears that 5.56mm ammunition could be used in handguns involved in crimes, the ATF sought public comment on the proposal.

The National Rifle Association and other gun-rights advocates flooded the agency with comments, leading to this week's decision not to act on a possible ban.

Murphy said he believes the agency caved to the gun lobby.

"I really have no other way to explain ATF's behavior there other than it gave in to pressure from the NRA,” said Murphy. “They publicly said they were shutting down the comment period in part because they received so many emails and so many letters directed at them from the NRA."

The rounds could be used in the AR-15 rifle that was used in the 2012 massacre at Sandy Hook Elementary School.

A spokesman for the National Shooting Sports Foundation said a pair of national law enforcement groups – the Fraternal Order of Police and the National Sheriffs Association – have said the 5.56mm rounds don’t pose any additional risk to law enforcement.

Mike Bazinet with the NSSF said more than 200 members of Congress from both sides of the aisle opposed the ban on the ammunition.

“We would encourage the senators to work to address the true threats posed to our law-enforcement officers by criminal activity, instead of initiating news-cycle timed activities that do nothing to promote public safety” Bazinet said in a statement.

The NSSF also pledged to work with the ATF on the now-withdrawn proposal.

Murphy disagrees with that position. He and Blumenthal describe the ammunition as “cop-killers” and say Connecticut as a state has spoken on an issue it hopes will be settled at the federal level.

"Given the fact that we've banned them here in Connecticut, I haven't had one gun owner tell me that they haven't been able to hunt or shoot for sport because they don't have access to armor piercing bullets,” said Murphy.