There's a few myths floating around in this thread. Especially this one.....
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With a bolt gun, if you're very careful on overall cartridge length (case stretch at the neck), you can neck resize only for 3-4 uses. By doing that, you're using fire-formed cases and you're accuracy will improve very slightly because the case is fitted to the chamber.
Maybe you'd like to explain why. Cases stretch. Depending upon the type of case, they stretch in different places. 45-70's stretch at the base (web) and you will finally get a crack at the web after reloading the brass a number of times.
338 brass stretches over the entire length of the cartridge. I would guess because the round is held in the chamber by the belt. 6mm seems to stretch (in my rifle) mostly at the neck.
The point being - the brass gets longer as you shoot it so if you reload it and crimp at the same point (top of the neck) without trimming the case, the overall length of the round is longer as the brass is reloaded a number of times. At some point, it will not cycle reliably in a semi-auto even though it still fits in the magazine - at least not in my semi-autos when I've tried repeated neck resizing instead of full length resizing.
As for fire forming - I can only relate my experience. Two of my rifles exhibit slightly better accuracy with neck resized ammunition than with full length resized ammunition when shot from a sled comparing the same load. The accuracy improvement is very small - but repeatable with my rifles.
The last thing I will say on accuracy is that I have also found that the concentricity of the round has a great effect on accuracy. I chased an accuracy problem for nearly 3 months with my .308 AR using factory ammunition. When I started measuring every round for concentricty I found the factory ammunition to vary as much as 0.009 between rounds.
I started sorting the rounds into groups by how close to zero they measured. I then loaded 5 rounds of 0.001 - 0.003 and tested the rifle with two 5 shot groups. The average of the two groups at 100 yards was .428. When I took the remaining rounds and randomly loaded them in 5 shot batches - the main grouping opened up to nearly 0.75 inches not including a flyer in each group that would have opened the groups up to nearly 1.25 inches.
I don't do "myths" or repeat "myths" - I go by my experience which, granted, is emperical testing and may not be repeatable by anyone else - but, that's the only information I can relate is my experience with a subject.