Best tank engines of WWII

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The US M5 3" gun was similar in size and weight to the US 90mm or British 17 Pounder. the 76mm M1 was made to duplicate the 3" performance in a lighter weapon. The US trialed the 76mm in the original M4 turret, and found it too cramped.

But there was a way. Israel picked up Shermans after 1948 from a vareity of sources, and began upgrade programs, that was easy, given the modular construction.

leftover from the 1973 war, the M50. Note original image has been flipped, here is proper orientation


This was an early M4 direct vision hull, upgraded with the E8 suspension and Wright Radial replaced with a Cummins Diesel

The turret was an original 'low bustle' but had a loader hatch added, a counterweight welded to the backside to balance the new heavier main gun, the French GIAT CN-50 L/61 75mm, a gun built to match the German KwK42 in performance.

Much more powerful than the original M3 75mm gun, a box was welded to the original mount opening, allowing the gun to be moved forward outside of the turret ring, a heresy that the US Armored Force abhorred from the balancing issues it caused

This was an improvement on what the British had done for similar reasons with the Firefly's 17 Pounder

by welding a box at the rear for recoil travel and retaining the Radio Gear in the Turret, per British practice
 
This was an improvement on what the British had done for similar reasons with the Firefly's 17 Pounder
View attachment 762736
by welding a box at the rear for recoil travel and retaining the Radio Gear in the Turret, per British practice
Your description of the layout of the Firefly is a bit misleading.

The recoil of the 17pdr was contained within the limits of the original turret ring. A hole was then cut in the rear of the turret and an armoured box fitted to contain the radio, the weight of which helped balance the turret. Some vehicles, as in the artwork above, then had a light metal storage box attached to the rear of the radio box. That storage box was a common feature of ordinary Shermans in Commonwealth service, attached to the rear of the turret bustle.

Because the larger 17pdr breech obstructed the loader's exit route via the commander's hatch, a new rectangular hatch was cut above his position on the left hand side of the turret roof, on those earlier production base vehicles that did not already have the oval loader's hatch in that location.

Photos of Firefly turrets here (none with the storage box):-

And with the storage box:-
 

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