During World War II, mine shells would see a resurgence as the Germans started to use the type in small caliber (initially 20 mm) automatic weapons, both to arm the Luftwaffe's fighter aircraft and for Flak. This was an innovation, as prior to this, mine shells had only been constructed in large calibers for technical reasons. Larger shells were usually produced by casting, smaller calibers by drilling the cavity for fuse and explosives into a solid steel shot, and neither process was effective at making small projectiles with walls that were sufficiently thin yet strong enough to work as a mine shell. While small thick-walled shells fired from automatic guns performed well against ground targets, they were more limited in anti-aircraft use.
