It's a cartridge where the shoulder diameter is greater than the head size. It has to be insertred into the chamber backwards, andd hammered into place. Extraction is a bear!

Answer here, of course, depends on what kind of varmints youse is talkin about. For bears, I swears by the .270. Fer loose rhinos, I chooses a .257 Weatherby Magnum. This is based on the recommendations of none other than ol' Roy Weatehrby hissef. Also the .25/50 single-shot (not Remintun).