Re: What is "neck" on a rifle spec?
I believe it refers to the actual diameter of the chamber where the neck of the cartridge will sit.
My understanding is that best accuracy is often from a tight chamber, but "tight" doesn't really mean anything without a reference. Specing a neck size in your rifle allows you to run a desired neck size on your brass and ensure a close fit.
I just know the theory a bit you'll have to wait for more knowledgeable folk with real experience if you want practical advice on actually choosing a neck size.