The 22 ARC uses a necked-down 6.5 Grendel case with no other changes (both based on the 7.62×39mm). The 22 ARC was standardized about 2020. The 22 ARC currently has two pressure standards, depending upon whether it is to be used in an AR or a bolt-action rifle. See closing paragraphs. Performance claims notwithstanding, when chambered in an AR, the 22 ARC is limited by the pressure the gun can safely tolerate when using a case with the body diameter significantly larger than the case used for the 223. Even with a 24-inch barrel, safe top-end loads in an AR cannot match what the 22-250 can do. The difference is significant. In a bolt-action rifle when loaded to the same pressure as the 22-250, the difference is perhaps modest but it still exists. Time will tell how the battle between the 22 Valkyrie and 22 ARC, as varmint chamberings for the AR settles out. I suspect it will come down to marketing more than any significant difference in actual performance. Just as with the 6mm ARC, the concept of having two pressure standards for the same cartridge, depending upon the gun it will be used in, is problematic. Our ancestors went through the same thing 100 years ago when Winchester offered High-Velocity loadings of the 32-20, 38-40, and 44-40 cartridges, for use ONLY in the Model 1892 carbine, rifle, and musket versions. It also offered High-Velocity loads for several larger cartridges to be used ONLY in later, stronger Winchester guns that would safely handle higher pressure than the original blackpowder loads generated. Inevitably, shooters used those loads in older and weaker guns with exactly the outcome one might expect. It is hard to see how even suggesting two pressure standards for these modern chamberings, depending upon the gun used, can possibly have a different outcome. Folks will overload ammo used in the ARs, which will lead to fatigue and failure of the bolt or barrel extension and people will get hurt, this seems inevitable.