Break-open with extractors for reloading, double action trigger with rotating firing pin selector.
The COP 357 is a 4 shot Derringer type pistol chambered for the powerful .357 Magnum round. It was designed by Robert Hillberg, based on his earlier work on the Hillberg Insurgency Weapon. It was manufactured by the now defunct COP Inc. of 3040 West Lomita Blvd., Torrance, California (COP stood for Compact Off-Duty Police). The double action weapon is about twice as wide, and substantially heavier than the typical .25 automatic pistol, though its relatively compact size and powerful cartridge made it a good choice for a defensive weapon or a police backup gun.
Drawing from US patent 4,407,085, covering the COP Derringer operating mechanism
The COP 357 is quite robust in design and construction. It is made of solid stainless steel components. Cartridges are loaded into the four separate chambers by sliding a latch that "pops-up" the barrel for loading purposes, similar to top-break shotguns. Each of the four chambers has its own dedicated firing pin. It uses an internal hammer, which is activated by depressing the trigger to hit a ratcheting/rotating striker that in turn strikes one firing pin at a time. Older "pepperboxes" also used multiple barrels, but the barrels were the part that rotated. The COP 357 operates similarly to the Sharps rimfire pepperbox of the 1850s, in that it uses the ratcheting/rotating striker, which is completely internal, to fire each chamber in sequence.
Two frequent complaints about the COP 357 are that it is too heavy to be used as a backup gun, and that the trigger pull is too heavy for rapid fire—even heavier than most modern revolvers.