Technically, the M4's design was capable of handling larger guns than the 75 mm and 76 mm guns with which they left the factory. The British fitted Shermans with the more powerful Ordnance Quick Firing 17 pounder (76.2 mm) gun, a variant known informally as the Firefly. By the time of the Normandy campaign, the M4 had become the workhorse tank of the Allied forces. Some M4 Mediums were equipped with the Duplex Drive system (Sherman DD), which allowed them to swim using a collapsible screen and inflated rubber tubes. Along with this were the M4 Dozer (an M4 with a bulldozer blade), the T34 Calliope (mounting a multiple rocket launcher above the turret), the M4A3R3 Flame thrower (flame tank), and the Sherman Crab Mark I (a M4 Medium with a mine flail), as well as many other variants.