I'll try an analogy.
While tightening a connection fastener (a bolt), and you reach a point where turning force no longer goes up.
Is it a good idea to continue turning?
Does it make a stronger connection to stretch the bolt shank well passed it's yielding point?
Answers: No, and No
I'm sure most here have experienced this in their lifetime. You just know you're screwed right there, and hoping with all you got that you can get the bolt back out (to replace it) without the head snapping off..
A common bolt connection is in proper tension while it's bolt shank has stretched, but not so far as to yield. When you loosen such a connection the bolt recovers it's form and can be re-used.
Where you go beyond this, the bolt yields, the tension it provides is not going up much, or at all, or is going down, and it will never recover it's form.
Back to reloading.
When you seat bullet bearing into excess interference fit, you cause neck brass yielding(up sizing). While there is high frictional forces required to do this work, the tension that grips the bullet (squeeze force) is not going up. And when you then pull that bullet, you find that the neck does not recover. It only springs back ~1/2thou,, it's normal amount, even if you had overcome fully 5thou of excess interference.
All you're doing with that is over-working the neck brass, while gaining no 'extra' tension.
To understand the futility in this you need to understand that bullets are not pushed out of necks on firing. Friction means nothing in this.
Instead, bullets are released from necks by their expansion, and even a relative few molecules worth of expansion fully frees the bullet.
That firing expansion is resisted by the neck's springback force times the area of bearing that force is gripping (PSI). Doesn't take much.
If pressure did have to overcome neck friction, internal ballistics would be completely different than it is.
In fact, if you were to force this, let's say by having zero neck clearance(not interference, but truly zero clearance), your gun would explode.
That is, unless you greatly reduced the friction to prevent this(so much so that your bullets would never hold set seating depths).
On our scale of things, including timing, it's hard to picture but very real. And you can test it & prove it (without dying).
Ideal mandrel use is a 'pre-seating' operation.
The mandrel would be at cal/bullet diameter. It would perform any neck upsizing so that your bullets don't have to (they're terrible expanders).
When you remove the mandrel the neck will spring back (inward) ~1/2thou, ready for actual bullet seating.
There is no difference in this from what you already do with your bullets.
To adjust your tension go to FORCE X AREA [pounds of force per square inch].
Adjust the LENGTH of downsizing to spring back against bearing (with that force of springback).
it seems my cold bore shots are closer together when I've been able to get my group size lower. Please explain the point you're trying to get across, because I'm really not understanding what this has to do with the OP's question.
