I have recently acquired an empty cartridge case, I believe of US origin, for 20 mm telescoped ammunition. The case is a grey metal cylinder 42.4 mm (1.67 inches) in diameter with a seam down its length. There are two end caps with holes for a screw-in primer and the projectile. The whole case including end caps is 140 mm (5.5 inches) long. The case is lightly magnetic, the end caps strongly so. There are no markings of any kind on it.
Can anyone provide me with any information about this? I am interested in the project which led to this being made, the gun it was designed for, when it was made and by whom, and how it performed.
Thanks for any help!