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They had that tank destroyer with the 76mm gun, can't remember its designation, which wasn't too bad.
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Are you talking about the Sherman variant? I believe it was called the Firefly (I think).
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The Canadians fitted the 76mm HV gun to the Sherman and it became the firefly. It was the only Sherman that stood a chance against a German tank one-on-one, but range was still less against a Panzer than the 88mm was on a Sherman (i.e. the Sherman was outranged). Later US tanks were fitted with the same gun in the M4A3E8 model. The low numbers of Shermans fitted out this way was mainly due to a shortage of production capacity for the guns.
The US used tanks differently back then. The idea of the US tank was to support Infantry. The 75mm was fine for that. Before the war, the Armor branch brought up the idea of putting bigger guns on them that would be able to take out other tanks, the Infantry board objected that if the tanks were so equipped, they'd run off hunting tanks instead of supporting Infantry, which was their job. The Armor branch shelved the idea. US doctrine at the time called for SP anti-tank guns to destroy tanks.
Later during the latter part of the war, enough officers with combat experience had been promoted to positions where this could be changed, and the 76mm guns were fitted, and the M26 came out. At the same time, the US began using tanks like the Germans. Before that, Patton was the only one who actually employed tanks like the Germans.
The tank destroyer was supposed to be the main Anti-tank vehicle of the US Army at the begining of the war. At first it was a half track with a 76mm gun on it, and was first used in Africa. It was dubbed the "Purple heart truck", and obviously wasn't very successful.
The M10 tank destroyer was a Sherman chasis and an obsolete 76mm AA gun in an open turret with a thin armor hull. It was a substitute standard vehicle, though many were made. The M18 Tank destroyer was the actual standard vehicle, and also mounted a 76mm gun, but used a different chasis. It also had very thin armor and no roof on the turret, but was a very fast vehicle. The M36 was the M10 fitted with the 90mm AA gun. This was probably the best overall tank destroyer of the war, and also the most versitile. The 90mm gun was a good gun, and also could be used in the indirect fire mode as on-call artillery, or even in an anti-aircraft mode in an emergency. Indirect fire was taught in the tank destroyer course at Camp Hood (now Ft Hood), and the M36 was quite effective and responsive as basically SP artillery. In the end though a M26 could do anything a M36 could (same gun) with fewer losses because it had real tank armor.
The major flaw in US doctrine was the assumption that the Germans were not as integrated, combined arms wise, as they truly were. The idea that think skinned AT guns mounted on tracks could destroy a combined arms force alone proved false. While one-on-one a tank destroyer could deal some hurt out, they never saw individula German tanks. It was always as a combined force, which resulted in the US using a combined force (infantry/tank) to oppose them. The end result was the Sehrman going up against the Tiger with the obvious outcome. When tank-destroyers were present, they could actually support the tanks, which were supporting the Infantry, and that worked to a degree (basically using the Shermans as bait and to keep the Tiger busy shooting things until they could pop the Tiger). The problem with that was that tank destroyers weren't organic to the maneuver units. They were owned by the Corps and delt out as needed.
Tank destroyers were phased out once we settled in with the doctrine of using tanks against enemy tanks.
Ross