My oddest caliber I guess would have to be 6mm remington. Not that odd but it is obsolete. I used to shoot factory ammo in it and it wasn't super hard to find, but I load for it now. The only other obsolete caliber I own is .35 remington. I also have a 16 gauge shotgun that is hard to find shells for at times.
I currently shoot the 6X47 Rem, .250 Savage, .300 Savage, .338-06, .358 Win, and .35 Whelen. I have owned at one time .223 AI, 6.5-06, .280 GNR, 8mm-06, and .375 JDJ. The short list of cartridges I want to try .358 Norma or STW and 6-204 Ruger and maybe a 6mm Dasher or XC.
7.92X57 (8MM Mauser) is my only "odd" caliber weapons. Performs roughly like a 30-06 and can be had for about the same price from Prvi-Partizan. I do worry that ammo may not be available someday...
I'll answer in the context of factory loaded cartridges for guns I occasionally shoot and which cartridges are not always available on dealer's shelves:
.221 fireball
.220 swift
.250 savage
.303 british
.38 S&W
.45 auto rim
reading through this thread it's kindof interesting what some people consider odd. 8mm mauser, 22 hornet, 303 british... those are just about as mainstream as sporter cartridges as you can get.
reading through this thread it's kindof interesting what some people consider odd. 8mm mauser, 22 hornet, 303 british... those are just about as mainstream as sporter cartridges as you can get.