"The Orlando and San Bernardino mass shootings, especially when viewed alongside similar carnage in Paris, make clear that individuals inspired by terrorist groups have eagerly adopted the military-style semi-automatic rifle, capable of shooting multiple rounds of bullets quickly and accurately, as a tool to produce maximum fatalities, mayhem, and fear. We are almost certain to see more such attacks, and as the Orlando event illustrates, they are extremely difficult to prevent, even when a person has been under suspicion, in part because they can be carried out without significant advance coordination or planning."