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.