Originally Posted by Mikecr
Bart, the problem with a 'bullet release force' measurement is that it does not actually represent 'bullet grip' , or tension. In fact, if not controlled, it might mean nothing at all.
Mike, I’m thinking static rather than dynamic conditions. And I’m also referring to the force needed to push the bullet out of the case when it’s fired, not the force needed to seat the bullet in the case measured with with any electronic or mechanical tool as you mention. To say nothing of the surface characteristics of the case mouth and bullet jacket which will cause more or less resistance to moving the bullet for a given compression force (neck tension?) the neck walls put on the bullet. Nor is the issue with cases with unturned necks with thicker neck walls at the shoulder and thinner ones at the mouth which is often the way things are. Besides, the ammo making industry typically uses “release force” as the physical stuff needed and it’s what most folks easily relate to and can easily measure themselves with inexpensive tools.
Info from another forum on this subject.......
A fellow who had participated in the HP White tests in the 1970's said they'd found bullets didn't start moving until about 10,000 psi. QuickLOAD thinks it's more like 3600 psi. Force on the bullet's equal to pressure in psi times cross sectional area of the bullet. Pressure increase partly stalls as the volume expands as the bullet slips from case neck to throat. There's at least one plot in the 1965 Lloyd Brownell study (link below) showing pressure flats at around 12,000 psi. So this can vary.
In the dynamic system, even though bullet pull is only 60 lbs slipping from the neck under the kinetic friction coefficient, given the short length of time the bullet has to start moving, its inertial resistance is a significant addition to that. The result is that instead of graciously popping forward, the neck is actually expanded from the rear by pressure, rolling forward almost to the case mouth before the bullet is fully released. This is the reason the mouths of fired cases are usually curled inward a little bit as compared to the rest of the neck. The rest of the neck expanded, but as the expansion neared the mouth, gas started leaking past it and equalizing pressure on the other side, so the mouth ceased expanding. We know this begins before the bullet has obturated the bore because super high speed photos show gas and powder particles preceding the bullet at the muzzle. Dr. Brownell attributed the effect of seating distance off the throat to the amount of greater gas bypass the deeper seating allowed.
Once you get to the rifling, Harold Vaughn showed 6mm (or .270) need around 1200 lbs of force to swage them into the rifling. That was about 20,000 psi static pressure equivalent, but assuming the kinetic coefficient of friction is about half the static coefficient of friction, about 10,000 psi would be right under dynamic conditions and with the bullet already moving.