I knew you were going to ask that. I'll have to check my gun rags to see if I can find the references. I read about wildcats, don't shoot 'em.
I did find that a German by the name of Gherlich invented the principle in the 1920s. Apparently more than just the Germans were fiddling around with this idea. Hotchkiss was even working with the French on a taperbore cannon. Most applications that I have ever read about were for anti-tank and anti-aircraft artillery.
I have also read about modern rifle makers using a micro-taperbore. Probably just an adjunct to a true taperbore that tapers ~30% of bore diameter.
...now the other experiment might have been gain twist rifling. But that's another story.
The British Army employed the taper bore principle called the littlejohn adapter which was a squeeze bore adapter attached semi permanently to the muzzle of the 40mm 2 pounder AT gun. It was used chiefly on Daimler armoured cars. These were employed for reconnaissance. If discovered or confronted by tracked enemy armour they would use their superior speed to escape or they might engage targets of opportunity using the impressive penetration capability.
The Germans used them to make very light AT guns. From memory the 75mm squeeze bore was capable of some 170mm or so penetration at 30 degrees at 500m which is unbeatable for a sub 1000kg gun. Competing guns would need 2.0-2.5 tons if firing APDS. Maybe 4 tons if firing a normal round. Because they were short of tungsten they stopped production of the weapon though continued to supply the ones that had been produced.
The Germans did have APDS but because they lacked tungsten they used them with ordinary materials which reduced their penetration. Early APDS was a mixed bag. Uneven and finicky separation of the sabots could reduce the accuracy so much it nulled out completely the potential accuracy increase of the flatter trajectory. Ordinary AT shells had a wind shield and a soft metal between the pentrator that welded and stuck to the armour and swung the shell around over coming partially the effects of sloped armour. APDS lacked this and was prone to bouncing and deflecting. That's why the US Army never deployed APDS. The German Army had APDS under development or TS "Triebs Spiegel" ammunition under development for their 50mm L/42 guns used on the Panzer III. The failure of this program to reach production in time gave them a lot of misery when they confronted the T34 during Barbarossa.
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