If you neck size then you gain the full benefit of fire forming.
For this to be true, then you have to neglect all of the inconsistencies neck-sizing only brings with it.
What we have not discussed in this thread is the fact “fire forming” doesn’t actually perfectly form the case to the chamber, because the brass has memory and springs back slightly. In most instances, fired cases will still fit and lock back into the chamber without interference or “crush fit.” So with one firing since body sizing, neck sized only cases will still fit into the rifle. But…
We only get away with that for a few firings - and the case is changing with every subsequent firing. When we fire multiple times without resizing a portion of the case, that brass “forgets” it’s memory and eventually fits the chamber exactly - which some folks unfortunately are misguided to believe is a good thing. Because we can’t actually put on a glove which is the exact same size as the skin on our hand…
Using my example from page 1 here - but expanding for additional detail: virgin brass is 15, what actually fits (kiss length) into the chamber is 20, the chamber is actually 21. We fire once, and the case still fits = 19. We neck size, fire again, and now the case becomes a 20, maybe I can still close my bolt, but I feel the slight crush or drag on feeding and closing. I fire again, and the case becomes a 21. Upon my next loading, the round won’t close into the chamber because the case has grown to an interference fit. So now I’m on the clock at a competition or I’m stalking the bull of a lifetime, and my rounds won’t close into battery…
But oh, I’ll just FL size every 2-3 loadings to “reset the clock,” and I won’t ever have chambering issues… Ok, great, except that you’re still committed to inconsistency in case volume for every firing - first firing is 15, then second is 19, then 3rd is 20, then resize to 18, then the second 2nd is 19 again, then second 3rd is 20 again, then resize to 18, then 19, then 20, reset…
So what consistency do you gain? Nothing. It’s fallacy. Setting your sizing die to minimally resize the entire body upon every firing is more consistent - 18 every time…
It is true and fair to point out the fact not every dimension of every mass produced die will fit every dimension of every rifle chamber - and by extension, true and fair to point out that our best opportunity for best brass-to-chamber fit is accomplished by custom reamed dies made by the same smith that chambered the barrel (different reamers, however, despite common internet misconception). But we know we can promote improved consistency by starting with the same case volume every firing than we can by allowing cases to expand inconsistently from one firing to the next, to the point of requiring a reset button every few loadings…