I so badly want an excuse to build a 22 PPC... I know I shouldn't, but I SO BADLY want to...
But to the point of the thread...
All we need with our sized ammo is for the case to 1) have consistent and concentric presentation of the bullet to the bore, 2) have consistent internal capacity, 3) have consistent grip on and subsequently offer consistent release of the bullet. We WANT cases to last a long time, so we WANT relatively little brass movement during firing and subsequent resizing, so we don't want our dies to be too much smaller in any dimension than our chambers, and to promote #1, we can't have TOO much free clearance in a few critical places - but overall, tapered cases with conical shoulders and proper free neck clearance should center themselves to the chamber axis and present the bullet coaxially to the bore. It's not really that complicated beyond that.
Competitive formats went away from neck-sizing only because it's not actually better for accuracy. Guys broke all of the records set by neck sized ammo, and guys beat the dudes who were neck-sizing only, and competitors remembered why they shouldn't only neck size. It was a fad for a while, and for some reason it hangs on based on individual anecdotes like a low volume shooter needing 8yrs to put 10 firings on a set of brass... A lot of the credit for popularizing neck-sizing only is given to McMillan's world record in the early 1970's and it DID stand for a long time. But it WAS beaten - and by a guy who was full length sizing. And it's not just ONE record out there which defines "what is most accurate" - there are a whole lot of records being held for various rifle classes and distances, and many of them are getting broken repeatedly as time goes on, and matches are getting won and lost every weekend. If only sizing necks was the route to best accuracy, every precision shooting sport would do it.
***I'll admit, I didn't know until today that Stinnett's 30 Stewart is/was a 6.5 Grendel variant, albeit not surprising, knowing what the PPC case can do, but I find it kinda interesting, since so many folks joke about the "30 grendel" potentially becoming popular, displacing 7.62x39 simply because a market is fickle, but hey, when a 30 grendel holds the "unbeatable record," maybe it's a thing...