▁British ▁use , ▁the ▁C rom well ▁and ▁Cent aur ▁design ▁requirement ▁was ▁changed ▁to ▁move ▁from ▁the ▁ 6 - p ound er ▁to ▁ 7 5 mm ▁for ▁common ality ▁of ▁am mun ition . ▁ ▁This ▁reduced ▁the ▁arm our ▁pen etr ation . ▁ ▁An ▁u pr ated ▁ 7 5 mm ▁High ▁Vel ocity ▁gun ▁was ▁designed ▁to ▁overcome ▁the ▁issue , ▁but ▁proved ▁too ▁large ▁for ▁the ▁new ▁t anks , ▁placing ▁a ▁renew ed ▁focus ▁on ▁the ▁ 1 7 - p ound er . ▁ ▁C rom well ▁was ▁first ▁used ▁in ▁action ▁with ▁the ▁Norm andy ▁land ings ▁in ▁June ▁ 1 9 4 4 . ▁ ▁A 3 0 ▁Ch allen ger ▁was ▁created ▁as ▁a ▁derivative ▁of ▁C rom well ▁to ▁meet ▁the ▁needs ▁for ▁a ▁ 1 7 - p ound er ▁armed ▁cru iser ▁tank , ▁but ▁production ▁was ▁cancel led ▁when ▁a ▁modification ▁to ▁the ▁Sh erman , ▁Sh erman ▁Fire fly , ▁proved ▁easier ▁to ▁produce . ▁This ▁allowed ▁tank ▁production ▁to ▁re - focus ▁on ▁C rom well ▁and ▁the ▁new ▁Com et ▁design . ▁ ▁A 3 4 ▁Com et ▁improved ▁on ▁C rom well . ▁It ▁mounted ▁a ▁further ▁upgrade ▁to ▁the ▁High ▁Vel ocity ▁gun ▁that ▁previously ▁couldn ' t ▁be ▁fitted ▁to ▁C rom well , ▁now ▁made ▁capable ▁of ▁firing ▁ 1 7 p ound er ▁( n omin ally ▁ 7 7 mm ) ▁project iles . ▁It ▁replaced ▁the ▁need