The Tank Thing.

In what may become a weekly event I present: The US M18 Hellcat
Armament: 1x 76mm (3in) gun, 1x 12.7mm .50 Caliber machine gun.
Armor: up to 12mm
Crew: 5
Weight: 17.9 tons
Speed: up to 50mph
Range: 150 miles

The M18 entered production in 1943, as an open turreted tank destroyer. A vehicle with a heavy punch, high maneuvering speed but little armor. The Hellcat first saw service in Anzio Italy and was used all over northwest Europe later in WWII. Smaller, lighter and faster than the M10 Wolverine Tank Destroyer the M18 relied on speed and agility and the heavy hitting power of it's 76mm gun. The 76mm gun however did not fare well against the heavier German tanks, such as the Panther and the Tiger. The development of the High Velocity Armor Piercing round (HVAP) helped mitigate the issue, but the round was never available in great quantity.

All in all the Hellcat was better in theory and on paper than in practice. High road speeds did not help much in open country manuvers and by the late war era the gun was just under powered.
Had it been developed a year or two earlier in the war it would have made a much bigger impact in my opinion.