5mm/.20 cal Winchester???

[Old Muley]

I have recently come into possession of some unfired, straight-wall rimfire cases I thought were .22 Win Rimfire Magnum, but after (purchasing and) measuring, they turned out to be 5mm/20 cal. and very loose in a .22 mag chamber. They are marked with the Winchester Super X head stamp. Is anyone familiar with this cartridge? Was it a failed experiment? To my knowledge, the cartridge never came to market. Does anyone have chamber dimensions? Can send cartridge dimensions on request. Thanks to all who can help!

[DanishM1Garand]

A neighbor had one. I found ammo for it 20 years ago. I'm out of the loop for decades on this cartridge. It is an orphan cartridge from years ago.

[Mk42gunner]

Straight walled and .20 cal? I wonder if it isn't a power load of some sort.

Robert

[Texas by God]

How do the measurements compare to a .22 Long Rifle fired case?

[Eddie Southgate]

The 5mm Remington rimfire round I am familiar with was bottle necked .

[kr54]

There was a target 'game' about 40 years ago. It used ice balls as targets and I think the name was Wingo. I believe it used a nonstandard size bore , very fine shot and a smoothbore gun. It was promoted by one of the big manufacturers and was inside, kinda like bowling. It wasn't something you could do in the back 40. I read about it in a gun magazine eons ago.

en.wikipedia.org/wiki/Wingo