Armament
The gun on the original M4 was the short-barreled medium-velocity 75mm M3 gun. When it first saw combat in North Africa in late 1942 against the Panzer III and Panzer IV, the Sherman's gun could penetrate the armor of these tanks within 1,000 yd (910 m). U.S. Army Intelligence discounted the arrival of the Tiger I in late 1942 and the Panther tank in 1943, predicting the Panther to be a heavy tank like the Tiger, and doubted they would produce many. There were also reports of relatively small British 6 pdr (57 mm) guns being able to take out the Tiger. However, this was only happening at very close ranges and against the thinner side armor. Due to their misconceptions related to this, and also due to tests seeming to prove the 76 mm able to take out the Tiger and the Panther without worry, AGF was not worried about the Tiger that much. These tests were later ruled inaccurate, with Eisenhower even remarking he was wrongly told by Ordnance the 76 mm could knock out any German tank. The Army also failed to anticipate the Germans would make the Panther the standard tank of their panzer divisions in 1944, supported by numbers of Tigers.
Despite the Bureau of Ordnance development of new 76mm and 90 mm anti-tank guns, Army Ground Forces rejected their deployment on U.S. tanks as unnecessary. Even in 1943, most German AFVs (later models of the Panzer IV, StuG III, and Marder III) mounted 7.5 cm KwK 40. As a result, even weakly armored light German tank destroyers such as the Marder III, which was meant to be a stop-gap measure to fight Soviet tanks in 1942, could destroy Shermans from a distance. The disparity in firepower between the German armored fighting vehicles of 1943 and the 75mm-armed M4 was the impetus to begin production of 76mm-armed M4s in April 1944. The U.S. 76 mm proved comparable in penetrating power to the 7.5 cm KwK 40, the most common German tank gun encountered during the fighting in France, however transfer of the tanks to the front started slowly, and most tanks still had M3's, even by Operation Cobra.
The 76 mm gun could penetrate roughly 88 mm of armor at 1000 m, just around the average tank engagement range noted by the Canadians. This was enough to reliably penetrate a PzIV's glacis. However, the 76 mm was not powerful enough against the frontal armor of a Panther. Due to its angle, the Panther's glacis gave it an effective thickness of 140 mm (5.51 in). Shermans had to get relatively close, due to both the armor and low-flash powder of the Panther which made it harder to spot. Sherman crews also had issues with firing from range as the Sherman's high flash powder made their shots easy to spot. Their gun sights were fixed magnification compared to the German's multiple magnification settings with added anti-glare filter. In Summer 1944, after breaking out of the bocage and moving into open country, U.S. tank units who were engaged at range from German defensive positions sometimes took 50% casualties before spotting where the fire was coming from.
The Sherman was first equipped with the L/40 75mm M3 Gun, which firing the usual M61 round could penetrate 77 mm at 100m and 61 mm at 1000m. The average combat range noted by the Americans for tank vs. tank action was 800m to 900m. Conditions later in the war necessitated up-gunning to the 76mm L/55 M1A2, which could penetrate 124 mm at 100m and 83 mm at 1000m using the usual M79 round. The M1A2 helped to equalize the Sherman and the PzIV in terms of firepower, although the M4 was still under-gunned compared to the Panther's much more powerful L/70 75mm gun, which could penetrate 138 mm at 100 m and 111 mm at 1000m using the usual PzGr.39/42 round. The British-developed Sherman Firefly was an M4 re-gunned with their QF 17 pounder anti-tank gun. The 17 pdr was a 76 mm gun and had a 55 caliber barrel, but introduced a much bigger charge which allowed it to penetrate 140 mm (of RHA sloped at 30 degrees) at 100 m and 120 mm at 1000 m using APC Mk.IV shot. This gun allowed the Firefly a slight firepower advantage over the Panther, although the muzzle flash due to unburnt powder from the increased charge left crews momentarily blinded after firing.
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