Where have you been the last ~70yrs?
The 220 Russian was the first major x39 variant, which then served to parent the 6 PPC, then the 6.5 Grendel, aka 264 LBC, which parented various common wildcats such as the 22 Grendel, 243LBC, 6mmAR, 6 Turbo40, 6 Grinch, 6 Rat/FatRat, 6 PredatAR, 30 ARX, 20 Grendel/20AR, etc, and now, has parented the new 6mm ARC SAAMI standardized cartridge. Some are pushed back, some are blown forward, some are just necked up or down... but guys have been wildcatting on the x39 case in almost every flavor imaginable for a long, long time.