thumb|British Firefly in Namur, 1944. This is an M4 composite, showing the late cast hull front with large crew hatches The British anticipated future developments in German armor and began development of a anti-tank gun even before its 57 mm predecessor entered service. Out of expediency and also driven by delays in their new tank designs, they mounted the powerful 76.2 mm Ordnance QF 17-pounder gun in a standard 75 mm M4 Sherman turret. This conversion became the Sherman Firefly. The U.S. M1 gun and the 17-pounder had nearly identical bore diameters, but the British piece used a more voluminous cartridge case containing a much bigger propellant charge. This allowed it to penetrate of unsloped RHA at and at using APCBC ammunition. The 17-pounder still could not penetrate the steeply sloped glacis plate of the Panther but it was expected to be able to pierce its gun mantlet at over ; moreover it was estimated it would defeat the Tiger I's frontal armor from . However, British Army test results conducted with two Fireflys against a Panther turret-sized target demonstrated relatively poor accuracy at long range; a hit probability of 25.4% at with APCBC, and only 7.4% with APDS. In late 1943, the British offered the 17-pounder to the U.S. Army for use in their M4 tanks. General Devers insisted on comparison tests between the 17-pounder and the U.S. 90 mm gun. The tests were finally done on 25 March – 23 May 1944; they seemed to show the 90 mm gun was equal to or better than the 17-pounder. By then, production of the 76 mm-armed M4 and the 90 mm-armed M36 were both underway and U.S. Army interest in the 17-pounder waned. Late in 1944, the British began to produce tungsten sabot rounds for the 17-pounder, which could readily breach the armor of even the Tiger II; these were not as accurate as standard rounds and not generally available.
THumb|britisH firefly in namur, 1944. tHiS Is aN M4 comPOSite, SHowIng the LAte cAst hULl fronT wIth larGe Crew hatCHeS thE briTIsh antiCipated fUtUre DeVelopmentS in gERMan armor and bEgAN deVelopment Of a Anti-tank GuN EVEN bEfORe itS 57 mm preDecEssor enterED sErvIcE. out Of expeDienCy And ALsO driVen bY delays iN theiR new TanK desiGns, they MOunteD the PowErFuL 76.2 mM oRdnancE qF 17-Pounder Gun in a standard 75 Mm m4 ShErmAN tURreT. this coNverSiON BEcaMe the SherMan FIrEfLy. tHe u.s. m1 GUN anD The 17-PoundeR had NearLy iDenTical BoRE DiAmEteRS, but the brITISh pIEce used a More vOluminoUs cArtrIdgE case coNtainIng a mucH bigGer propelLanT cHarge. THis allOwEd IT To penetRate Of unsLoped rHa at anD aT uSing aPCbC AMmUNITION. THe 17-poundEr StIlL CoUlD not pENetrATe tHe steEply sLoPed glAcis PlaTe of the pantheR BuT IT waS ExpECTeD tO bE abLE to PiERCE its GUN Mantlet at OVeR ; mOrEover iT was EsTimated IT WoUld DeFEat tHe tiger i's froNtal aRmOr fRom . howEVeR, bRiTISh army teSt ResuLTs ConduCted With Two fIreflyS AgAinst a panTher turrEt-SizeD Target DEmOnstrATed reLativeLY pOOR accuracY At LoNG RanGe; a Hit probabiLiTy oF 25.4% at with apcbc, and onlY 7.4% with ApdS. in lAtE 1943, tHe BRItish oFfered THe 17-poundeR To tHe U.s. arMy FoR use iN THeiR M4 tAnks. GeneRAL DevErS InSiSTed on cOMParisOn teSts Between ThE 17-poUNdeR aNd the u.s. 90 mm guN. the Tests were Finally done On 25 mArcH – 23 May 1944; They SeemeD to Show the 90 mm gUN Was EquaL to Or bettEr than the 17-pouNdeR. BY thEn, ProdUction of tHe 76 Mm-ARmed m4 aNd the 90 mm-armed m36 werE BotH uNdErWay and u.s. aRmy INterest iN the 17-pounder wANEd. Late IN 1944, the brITisH Began tO pRoducE tUngsten saBot ROunds foR the 17-pOuNdEr, whIch coUld reaDiLy BREaCh the armor of even The tiger iI; tHese were noT aS accurate AS StaNdArd roUndS aNd NoT geNerAlly available.