Standardized in 2020, the 6mm ARC follows significant experimentation with modifications to the 6mm PPC, which was derived from the 220 Russian case. The standardized version of the 6mm ARC differs significantly from what would, at first, appear to be the parent case, the 6.5 Grendel. Compared to the Grendel, the ARC has the shoulder driven back and the neck shortened slightly. The 6mm 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. Shooters should be aware that several wildcat versions of the same basic cartridge exist. Chambering and firing standardized 6mm ARC factory ammunition and handloads is possible in guns with those chambers. The potential for case failure is significant. The potential for that to result in a dangerous gun-damaging or gun-destroying gas leak with the further potential of generating injury or death to the shooter and bystanders is equally significant. If you have any wildcat rifle chambered in a 6mm round based on the 6.5 Grendel or 6mm PPC case, no matter what you might have been told the chambering name is, do not shoot 6mm ARC ammunition in it without first verifying it is safe to do so. You might well have to obtain custom dies to properly convert and reload cases. Lee Precision can help with this. Comparing performance of the 6mm ARC to performance if the 6.5 Grendel is enlightening. Not surprisingly, the former has the trajectory advantage and the latter has the delivered energy advantage. Recoil in either is modest but, the lighter recoil of the ARC could matter to some folks in some guns and situations. A plethora of components is available for handloading the ARC and it works well with bullets up to the 110-grain Hornady A-Max and the Berger 109 LR-HT. I have proven a quality AR, such as the one Craddock Precision built for me, can deliver consistent groups well-under half-MOA while launching such bullets at about 2700 fps using only about 30-grains of propellant. As such, when loaded with the right bullet, the 6mm ARC is entirely suitable for deer hunting. Just as with the 22 ARC, the concept of having two pressure standards for the same cartridge, depending upon the gun it will be used in, is problematic, to put it mildly. 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 of 1892 carbines and rifles. 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 ARs, which will lead to fatigue and failure of the bolt or barrel extension and people will get hurt, this seems inevitable. The short neck used on this case does no one any favors but the constraints of the AR-15 limit options in case design. With such a small case, moving the shoulder back to increase neck length would result in a significant loss of usable capacity and therefore a significant reduction in performance. Increasing case body diameter would require use of less pressure, which would entirely offset the advantage of increased capacity. So, it is what it is. The relatively limited case capacity is the only mitigating factor, which will likely allow for decent barrel life, despite the short neck. The folks at Craddock Precision report the Ackley Improved version of this round works perfectly in the AR. It offers a modest velocity improvement with better accuracy. Another example of what might have been.