[MarkSheppard]

I got data on the 80mm PAW 600 gun:

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Development of an alternative to conventional nfled guns began in Germany during the latter part of the Second World War and in the first instance led to the 80mm Paw 8H63, a light, towed, smooth-bore anti-tank gun which fired fin-stablised HEAT projectiles similar in shape to mortar bombs with a muzzle velocity of 520 m/s. Paw 8H63 began to be produced by the end of 1944 and some proposals were made to mount similar guns in armoured vehicles. However, their development in Germany came to an end in 1945.

After the war the idea of a low-pressure gun firing fin-stabilised HEAT projectiles was taken up in Belgium by the Mecar company in the form of its 90mm Energa gun. The latter was first tried during the 1950s on experimental wheeled and tracked armoured vehicles built in Switzerland by Mowag. Subsequently it was mounted in some of the Commando armoured cars produced in the United States by Cadillac Gage but the first large scale use of this type of gun did not come until its development in France, following trials there in the mid-1950s of the Mecar gun.

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So yeah, I think that a modified Pz IV turret with the PAW 80mm Paw 8H63 is feasible.

More data:

Production: 260 examples
Range: 6,197 meters

The 260 guns were produced between December 1944 and March 1945.The projectile looked very similar to the standard German mortar rounds. Krupp were also developing a 10.5cm version at the very end of the war. Known as the PWK 10H64, the prototype was stillborn.

As for the actual round fired, it was 81.4mm in diameter. The cartridge was a cut down 105mm shell casing on top of which a thick steel plate with a series of venturi holes in it. The "bomb" (for want of a better term) was a modified 81mm mortar round with a shaped charge warhead. It was attached to the 105 cartridge by a spigot that had a shear point on it.

When fired, the gases of the explosion were primarily trapped in the 105 cartridge and only about a third escaped through the venturi to shear the pin and fire the bomb. It was accurate enough to put 9 out of 10 rounds in a 1 x 1 meter square at 500 meters. Penetration was 145 mm; a performance sufficent to take out virtually any tank of the day.