Out of all the cartridges I've seen discussed here so far, I haven't noticed any for which brass/ammo isn't obtainable, at least on the part of a shooter with some determination and sleuthing skills. In that respect, have any truly vanished?
If you want one that truly has vanished, try finding brass/ammo for .25-20 Single Shot, and its progeny the .22-3000/R2 Lovell/.22 Maximum Lovell wildcats. It's been 70 years now since the last factory stuff was made, and cases from boutique brass makers have come and gone. A lot more rifles exist in those chamberings than you might imagine (I personally own three), and those of us who shoot them jealously hoard the precious brass. (It's of an odd dimension that doesn't lend itself to being made from anything else.) These things were everywhere back in the 30's-40's, a highly popular .22 CF varmint round. Then overnight the ammo vanished, almost literally. What killed it? The .222 Remington, a Remington success story that IMO makes up for their fumbling the 6mm/.280/.260 footballs.
"You can lead a man to logic, but you cannot make him think." Joe Harz "Always certain, often right." Keith McCafferty
