Tension and friction may correlate or they may not, depends on controls in-place.
It's important to understand that they are different separate things.
Neck tension is the neck spring back force squeezing against an area of seated bullet bearing.
Where you increase the area gripped, or the force gripping, you increase tension (T=FA).
We currently have no tool/method for measuring the actual neck/hoop tension gripping our bullets.
For tension correlation with friction, that friction must be taken to a consistent standard(removed from variance). It's easy enough to do this; clean bullets, leave the carbon layer inside necks, provide the same interference fit for each seating. Here, variances in seating force are caused by neck resistance to expansion, forced by a seating bullet. While the measure here includes sizing (upsizing with bullets), this is still not isolating tension alone. But it's useful.
If the interference provided is within normal neck spring back, say 1thou max, then you can correlate seating force(friction) with neck tension. This would not be linear though. Attempted conversion & graphing would get squirrely as bearing nears donut area or neck-shoulder junctions. It would get really ugly where neck sizing length goes beyond seated bearing(causing base binding). We're only led to do these things, and to use bullets for neck upsizing, out of bad planning.
It makes no sense to size necks down so much as to leave seating interference of more than 1thou.
The excess is only re-expanded by bullets, which are not designed for this.
You can measure a seated neck, pull the bullet, and see that it springs back 1thou at most from cal. That is what was holding the bullet,, not an excess interference,, regardless of the amount of excess interference.
It makes no sense to seat bullets into donuts or neck-shoulder junction, or beyond seated bullet bearing.
Any of these conditions will reduce control and increase tension variance.
To control neck tension and see it vary from round to round, downsize to provide no more than 1thou interference for seating (after outward spring back). Adjust the length of this downsizing, not to exceed seated bullet bearing. Leave the carbon layer in necks, use clean/consistent friction bullets. Carefully measure the seating forces.
With this, you'll see all matters of spring back variance, including softening/hardening and thickness variances. Set it, load develop with it, tweak it, manage it, and most important: understand it.
