I disagree. Your brass doesn't have any idea if it's being fired in a tight neck chamber or not. All it does is blow out until it contacts the neck of the chamber and then springs back some. So this idea that you only turn your necks if your chamber is tight just to make it fit in the gun is like trying to take an oral temperature by sticking the thermometer in the rectum. In other words, people didn't start neck turning to make brass fit, they started neck turning to make the brass UNIFORM in thickness. Of which the result is you make your brass thinner and therefore (for less work hardening) the neck diameter needed to be reduced to control the blow out of the brass and minimize the springback.
Why make the neck wall thickness uniform? Striaght bullet release. Pure and simple. It also makes the loaded round more concentric because the sizing die isn't squishing in a fat side of the neck anymore. These things are both benefits of neck turning regardless of how much clearance you have between the loaded round and the chamber walls.
Now, if you have a great barrel with an optimum oscillation, it may override the fact that the brass hasn't been turned, but turning would still make it shoot even better. What percent? 1%, 10%, 90%. Who knows. Some barrels won't show as much of an improvement as some others but the fact remains that wringing the best accuracy from any rifle requires all the variables be eliminated. And uneven brass is a variable.
