I think you are talking about the SPAT "Self Propelled Anti Tank" gun. It had vertually no armor but it could move very fast. When you fired that 90mm, it seemed to jump 10 feet in the air due to the recoil. It was designed to give the airborne an anti tank capability.
As far as the 106mm on the Ontos, I always felt like they could have designed a over caliber missle that you loaded from the front with the launch charge being in the tube and a substainer motor being on the missle itself. Of course you would have a guidance system in the missle section. If you didn't want to fire a $1,000,000.00 missle at a $10.00 bunker you could always use the conventional 106mm round.
The concept with the front loading over caliber warhead with a substaining rocket motor and explusion charge in the tube is in use today in a smaller form. It is called the RPG 7. The RPG is actually a recoiless tube launched rocket when you want to get techical. Its fore runner the RPG 2 only had the explusion charge and no rocket assist. For some reason with the name RPG everybody always called them rocket propelled grenades. The idea was gotten from the German panzerfaust. However the first use of an overcaliber round like this was employed by the Germans on their 37mm anti tank gun. They made a large shaped charge round that simply slid over the barrel of the 37mm anti tank gun and used something simular to a blank charge to launch it. The main anti tank gun of the German army when then went into Russia was the 37mm which becamed known as the "Door Kocker". The only use it had against the KV1 and T34 was to band against the side of the tank and let the crew know someone was out there. With the use of the over caliber round, they could kill those tanks but the tank had to be probably less than 50 yards or so away and the rate of fire was very slow.