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Can anyone identify this 20mm shell casing?

My grandfather gave me this shell casing, and I have been having a hell of a time trying to find out what kind of gun it is from.

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.

Picture of Can anyone identify this 20mm shell casing?
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Re-design5 years ago
Burf Re-design5 years ago
+1
According to my trusty ammo ID manual its for a Mark 4 naval deck gun. Primarily used in single or dual mounts (two guns) as an anti-aircraft gun by the U.S. and British naval forces during WWII.
onrust Burf5 years ago
Man, that thing don't even look user friendly! Gtaet answer
*great
caarntedd5 years ago
BTW, there are quite a few shells that are crimped to the projectile, especially the more powerful ones or those with a heavy projectile. Under recoil the unfired rounds in the mag are subjected to movement that can eventually work the projectile out of the shell increasing the overall length which could cause a feed problem or jam in the mag. Also rough handling such as travelling off road in a vehicle can cause the projectile to move in or out of the shell.
ilpug (author)  caarntedd5 years ago
Okay, thanks for the info. My uncle said it was rare for the shell to be crimped like that, but he probably isn't the most reliable source. That all makes very good sense.