Now that I think back, I was once warned about buying a "ballistic knife"

A ballistic knife is a knife with a detachable blade that can be ejected to a distance of several yards by pressing a trigger or operating a lever or switch on the handle. Spring-powered ballistic knives briefly gained notoriety in the United States in the mid-1980s after commercial examples were marketed and sold in the United States and other Western countries. Since then, the marketing and sale of ballistic knives to civilians has been restricted or prohibited by law in several countries.

In its spring-propelled form, the blade of a ballistic knife is theoretically capable of being fired to an effective range of around 5 meters (about 16 feet) at a speed of 63 km/h (39 mph). Ballistic knives using compressed air or gas propulsion to fire the blade can be made somewhat more powerful, and do not suffer from spring fatigue over time.

Similar to conventional automatic knives, federal law makes ballistic knives with a spring-operated blade illegal to possess, manufacture, sell, or import "in or affecting interstate commerce." This means they are illegal to import from outside the United States, as well as buy or sell over state lines, including possessing or making them with intent to sell over state lines. The federal law also makes it a separate crime to use or possess a ballistic knife during the commission of a federal crime of violence, with a minimum sentence of 5 years in a federal prison.

Federal law does not prohibit the possession, manufacture, or sale of a ballistic knife within a state's boundaries, and the individual laws of each state or territory must be consulted to determine whether possession, manufacture, or sale within a given state is legal (many states have statutes that regulate or prohibit the acquisition or possession of ballistic knives, and penalties vary from state to state). Like the federal switchblade law, an exception is made for sale to the United States Armed Forces within the confines of a contract, as well as possession by duly-authorized members of the Armed Forces in performance of their duty.

I know, way too much information, but this might be what you were asking.