Yea, you wonder why they were going to build it.

A bit on the PAW 600 high-low pressure gun:

This weapon is a smoothbore gun that was designed as a cheap expedient to provide infantry divisions with a decent anti-tank weapon.
The round it fires consists of a 105mm howitzer casing cut down to about half height with a heavy steel plate welded on the face of it. This plate has a series of small holes (venturii) drilled in it. A short spigot is fitted to the plate. On the spigot a modified 8.1cm mortar bomb is attached such that when fired it detaches from the spigot. The mortar bomb is fitted with a hollow charge warhead. There was nothing sophisticated about it. It was hobbled togeather from existing bits of stuff.
Accuracy in tests was 90% hits could be obtained on a stationary 1m x 1m target at 500 meters. At double the range hits on the same size target are largely blind luck. Well, that makes it better than a panzerschrek at least!

Also, 38.5 tons and no more than 14.5mm of armor?! That's alot of weight for paper thin armor.