The very important fact is that this interface value must be able to be repeated time after time for any type of shot consistency
So, I think we can/have vaguely defined what neck tension in. The interference fit between the bullet and case neck, also with regard to more obscure topics like “grab” (how grippy the interior of the neck is), “cold welding” (the interaction between two dissimilar metals), uniformity of neck wall thickness and it’s presence on the outside vs. inside of the neck, and elasticity of the neck (a factor of times fired/annealing regularity).
All of this is good and well... but how does a reloader enhance consistency across loads?
Cold Weld - Easy, either load close to the match, coat bullets or seat them long and reseat before the match/session.
Neck Wall Uniformity and location - you can measure this, and it is a variable you can control via neck turning and mandrels. How much variation is too much depends on accuracy expectations, and there probably isn’t any hard data on variation and it’s correlation to bullet release variation. We can all agree that a consistent radius on the interior of the neck is more beneficial than a consistent exterior radius (mandrels to push inconsistencies outward).
Elasticity - Also controllable via a suitable and equally importantly... repeatable method. No, your BIC lighter, nor your drill and a socket would be considered repeatable. For the crowd that anneals every “X” firings... Is iteration 2, 3 & 4 the same as freshly annealed necks if you anneal every 5 firings?
“Grabbiness”- Many things effect this variable. Dry tumbling vs wet tumbling vs no cleaning have an effect. Bullet coating has an effect. Neck tension, thickness and elasticity have an effect. Annealed vs not annealed for that particular iteration has an effect beyond elasticity. In my opinion, with everything else being ideal, this is the biggest wavering variable from one loading session to the next. Control this to the best of your ability by doing case prep the EXACT same every time. Brushing necks will help, as will a dry neck lube. Annealing every time will also help vs annealing every X firings.
Main point is this; that you may not have a grasp on everything that influences the total effect of your neck tension, but the more consistently you prep your cases, the better off you will be in consist neck tension and bullet release.
