It takes six to eight seconds to fire off a 30-round magazine like that used in the Newtown killings, said Joseph Green, a retired firearms instructor and agent of the federal Bureau of Alcohol, Tobacco and Firearms.

A well-trained shooter would need roughly 13.5 to 22 seconds to fire off the same number of bullets with six magazines of five rounds each, which are legal in New York.

A shooter needs 1.5 seconds to swap out each cartridge, and each five-round cartridge takes one to two seconds to shoot.

Killers facing stressful situations — such as a lot of screaming people — might need a bit more time to swap cartridges, Green said.

But he and other experts say swapping magazines means just a minor delay for determined mass killers.