Caleb's been getting funny looks from people.... when he mentions wildcats or "wildcatting." No, he's not going into the oil drilling business, nor is he talking about cougars, panthers, or mountain lions. A wildcat cartridge is any cartridge that's not mass produced by a commercial ammunition manufacturer. They're not obsolete cartridges, those that WERE massproduced but have since fallen out of production. Handloaders develop wildcat cartridges to fill a need that's not covered by an existing commercially produced round.
For example, the .2545 Sharps and .277 Wolverine are wildcat cartridges that were developed to fit into the AR15 platform and offer specific velocity / bulletsize combinations not covered by the existing .223, 5.56 or .300 Blackout loads. (There's also a wildcatter tradition of giving the cartridges clever or colorful names.) Caleb found the .2545 Sharps to be an awesome deer and hog cartridge when the lived in Louisiana. Bonus: his favorite bolt action hunting rifle at the time was chambered in .257 Roberts, so both of his hunting rifles used the same bullets.
Wildcats are often created simply by necking up or necking down an existing cartridge case to take a differentcaliber bullet. Often, the only change that needs to be made to convert a rifle to the wildcat is a new barrel. Once you get reloading dies for your wildcat (along with the necessary brass, bullets, primers and powder), you can set up your press to crank out as much custom ammo as you need.