Dubbed the "Mighty Mouse" the Mk 4 FFAR was 4 ft long and weighed 18.5 lb, with a 6 lb high-explosive warhead.   Four fins flipped out on launch to spin-stabilize the rocket. Maximum effective range was about 3,700 yards. Rockets were generally fired in large volleys, some aircraft carrying as many as 104 rockets.

FFARs were the primary armament of many USAF interceptor aircraft in the early 1950s, including the F-86D, F-89, and F-94C. They were also carried by the F-102 Delta Dagger to supplement its guided missile armament.

The Mighty Mouse ultimately proved to be a poor aerial weapon. Although it was powerful enough to destroy a bomber with a single hit, its accuracy was abysmal. Its spin rate was not high enough to compensate for the effects of wind and gravity drop, and the rockets dispersed widely on launch: a volley of 24 rockets would cover an area the size of a football field.

As a result, by the late 1950s it had been largely abandoned as an aircraft weapon in favor of the guided air-to-air missiles then becoming available. The Mk 4 found other uses, however, as an air-to-ground weapon, particularly for the new breed of armed helicopter. A volley of FFARs was as devastating as a heavy cannon with far less weight and recoil, and in the ground-attack role its marginal long-range accuracy was less important. It was fitted with a more powerful motor to become the Mk 40. The Mk 40 was a universal motor developed from the Mk 4 2.75 FFAR, and could be fitted with different warheads depending on the mission. Pods (typically carrying seven or 19 rockets) were created for various applications, and a wide variety of specialized warheads were developed for anti-personnel, anti-tank, and target-marking use.  The FFAR has been developed into the modern Hydra 70 series, which is still in service.