The ones I shoot that come closest to "obscure" in the Rocky Mountains are:
6.5X54 M/S
9.3X57 Mauser
9.3X74R
Some men and women say "never heard of that' when I tell them the caliber of these rifles.

The ones I shoot that I can't call Obscure, but are not at all "mainstream" anymore are:
222 Remington
300 Savage
303 Brit
8X57 Mauser
35 Remington
358 Winchester
404 Jeffery.
These are shells that everyone has heard of but they don't see used much these days and it's a bit odd to see someone using one.

I call a cartridge "mainstream" or "run of the mill" when if you tell someone what you have they don't see anything even slightly odd about the choice

My "run of the mill shells" are:
223
25-06
270 Winchester
6.8SPC
30-30
308
30-06
7.62X39
9.3X62
375H&H