Can anyone identify this 20mm shell casing?
On the bottom it says "20mm MK 4 I.S. 1944"
From my browsing on the internet, I think it might possibly be a US naval autocannon round, possibly from a 20mm Oerlikon gun, or from a British Antiaircraft gun
That said, I still haven't found an exact match. The thing that sets this shell apart from the others is the taper. Normally, casings are straight or are tapered in sections, at a much steeper angle. This shell has a simple taper at a very smooth angle. I have not found a shell that has a matching taper. The other distinguishing feature are the small divots around the edge, presumably to help hold the projectile onto the shell. This is rather rare, because usually the projectile stays seated in the shell without any special aid.
There is also a weld mark on the side of the shell, possibly from being held in a belt, but I am not sure.
I have big hand, so the shell looks rather small. It is actually pretty small for a 20mm shell.
Any help in ID'ing this shell is appreciated.