The Secret Service Counter Assault Team — CAT for short — is charged with fighting back if the president ever comes under attack. While the president's protective detail would be jumping in front of him and quickly getting him to safety, CAT is supposed to turn outward and "lay down an unbelievable amount of suppressive fire," an agent told The Washington Post.
CAT members are currently outfitted with the Knight's Armament SR-16 rifle, a variant of the military's standard issue M-4, according to the book "In The President's Secret Service."
Agents who are a members of CAT have to work their way up through the ranks of Secret Service before they ever got a shot in the agency's equivalent of special ops. There is a grueling training process, which includes many weeks of training that are both physically and mentally demanding.