Anti-Submarine Projector Mk 20 & 22 (Mousetrap)
    Though Hedgehog had been a success, its large dimensions, especially weight, caused much trouble in the fleet. In fleet destroyers, for example, one of the forward 5" mounts would have been due for removal if a Hedgehog was to be installed; this, among other reasons, caused Hedgehog in the U.S. Navy to be exclusively fitted to destroyer escorts and license-buld British frigates (and ASW refits of flushdeck destroyers).

    However, the benefits of Hedgehog could be of use in the large number of small subhunters, the PCs, which rarely even sighted a submarine but which made for good coastal escorts nevertheless. The new weapon, ordered developed early in 1942, was to fit on the small ships and yet be able to deliver an effective pattern of charges, 70ft wide. Those were originally intended to be slightly larger than the original Hedgehog round, but problems with the development of that new charge caused it to be abandoned in favor of the original Hedgehog round. A rocket (in this case meaning a slow-burning propellant) was attached to the lower body to make it fireable without the recoil inherent in Hedgehog. 
Mousetrap suffered from a far smaller pattern, lack of stabilization and range adjustment, and was not the equal of Hedgehog. It was, nevertheless, installed in large numbers aboard small craft and even on twelve fleet destroyers (because it could be mounted alongside the superstructure). These weapons were Mk 22 projectors, paired to equal a Hedgehog salvo, but was unsuccessful, probably causing the loss of the destroyer Turner in early 1944. 
Mk 20 was a four-rail projector, while Mk 22 added two rails on each side for a total of eight. Mk 22 was standard later in the war; Mk 20 being adapted to fit the additional rails. 

Usual range: 280 meters / 300 yards 
Mount Weight:  ?????lbs / ????? kg 
Sinking Speed: 7.6 m/s / 25 f/s 
Rate of fire: 1 per 3 minutes 
Charge: 65lbs / 30kg