Keep Americans Safe Act This bill amends the federal criminal code to make it a crime to knowingly transfer, possess, import, or bring into the United States a large capacity ammunition feeding device (LCAFD). It permits the continued possession of a grandfathered LCAFD. Additionally, the bill does not prohibit, with respect to an LCAFD: manufacture, transfer, or possession related to certain law enforcement efforts, or authorized tests or experiments; transfer or possession related to securing nuclear materials; and possession by a retired law enforcement officer. A violator is subject to criminal penalties—a fine, a prison term of up to 10 years, or both. Newly manufactured LCAFDs must display serial number identification that clearly shows the device was manufactured after this bill's enactment.