As Britain struggled to develop tanks capable of equaling the superior German Panzers, the decision was made to convert existing stocks of M4 Sherman tanks, delivered to the UK as part of the Lend-Lease Act. The so-called Sherman Firefly featured a long-barreled 17 pdr. anti-tank gun, that was much more effective than the original 75mm gun, capable of penetrating a Tiger I's frontal armor at over 1,000 meters using regular ammunition. The majority of Fireflies were adapted from M4A4 tanks, designated Sherman V by the British, while some were made using the earlier M4, or Sherman I in British usage. Vehicles rearmed with a 17 pdr. gun were given the suffix C, with the resulting units being Sherman VC and Sherman IC.