I picked this up in one of the bin sales at SLICS. It appears to be a brass shotshell with a brown wad varnished in the top of the case and a very thin rim, only .03" thick.
Neck Diameter: .372
Base Diameter: .378
Rim Diameter: .464
Case length: 2.202
Oval brass primer
Headstamp: W 17 at 12 and 6 o’clock
I don’t think it’s a shotshell (it doesn’t rattle when shaken). Somewhere in the back of my feeble mind I recall seeing something about this round many years ago, like it was maybe an artillery primer or something. I did a search here and it came up with 939 matches. Thanks for any help!