What is this shell?

[LeftyKid]

I picked this up at a local gun show in a bag full of miscellaneous oddball shotshells. I've managed to ID each of them... except this little thing. It's the smallest shotgun shell I've ever seen (not counting .22 shot).

It's a paper hull rimfire shell with a 5/16ths diameter (the brass being a hair wider) by 1 & 5/8ths tall. There are virtually no markings on it except for a plain "W" headstamp, which I assume stands for Winchester. After doing a little research I learned that .310 Remington skeet is a thing, but every picture I found of .310 ammo was made by Remington and had an all-brass hull.

It's been bugging me for a while, and clues would be appreciated, thanks.

 

[Mizar]

No. 3 Long shot (9 mm Flobert) for a garden gun.

Here is a link describing the cartridge:
http://www.ammo-one.com/9MM-SHOTGUN-SHELL.html

 

[1KPerDay]

That's cool!

 

[LeftyKid]

No. 3 Long shot (9 mm Flobert) for a garden gun.

Here is a link describing the cartridge:
http://www.ammo-one.com/9MM-SHOTGUN-SHELL.html

Looks like it. Googling "winchester 9mm flobert ammo" came up with this gunbroker ad from last month: https://www.gunbroker.com/item/759218823

Thanks, I didn't even know such a thing existed. Needed "flobert" to actually find them on Google. Everything else I could think of typing in came up with various other, more common, kinds of .30 cal Winchester ammo.

 

[entropy]

Here's a gun that shoots it...

 

[351 WINCHESTER]

First thing I thought of was the 9mm Flobert. When I was a young man an old gunsmith had one for his granddaughter. I saw the box of shells.