WWII America M18A1 20mm Cannon Dummy Round

Not sure if I would call it a cannon round but that is what it is called. While it looks very large compared to other ammunition it is not quite an artillery round. These would be linked like the .50 caliber machine gun rounds beneath it in photo #1 and fired from an Oerlikon 20 mm cannon. Those were used on US Navy ships for defense from aircraft. The US Army Air Force had 20mm cannon on some of its aircraft most notably the P38 Lightning. As can been seen it fires a rather large projectile. This one is 3 1/2" long. That is just the projectile portion. One would not want to get hit by that.
On the based it is marked "CARTRIDGE DRILL M18A1 20MM. GUN". Means it was used for practice. Probably used for dry fire exercises. Firearms can be damaged if a cartridge is not used when charging the weapon and dry firing. Anyway it is useful for practice even if the weapon will not actually be fired.
Now it just needs another 20-25 years and it will be an official antique.