Poland developed various different 20mm cannon in the late 1930s, only one of which got into service. All of them fired what looks like the standard 20 x 138B "Long Solothurn" round used in Rheinmetall-Borsig AA guns and various other weapons. However, it has just been pointed out to me that while the starting point for the Polish work was the German round, they modified it so that most of the dimensions are different. The case is longer, at a fraction under 140mm, but more importantly it is also about 1.0mm wider in the key dimensions (rim, belt, case body above the belt), and the case capacity is 3cc greater. See the details on this site: http://www.dws-xip.pl/wojna/bron/polska/p8.html This is enough of a difference to make the German and Polish rounds non-interchangeable.

Does anyone know any more about this? Did the Poles do this deliberately to ensure that German forces could not use captured Polish 20mm ammo? Presumably the German ammo could have been fired in the Polish guns?