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**Unclenick** · December 26, 2012, 04:59 PM

Slamfire,

Yep.

Bart,

You're thinking static rather than dynamic conditions. When I first joined this board a fellow who had participated in the HP White tests in the 70's said they'd found bullets didn't start moving until about 10,000 psi. QuickLOAD thinks it's more like 3600 psi, corresponding pretty well to the flat spot in Slamfire's pressure curve. The flat spot is due to the pressure increase partly stalling 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 showing 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.

Below is a photo of some cases I sectioned to show a well-used verses newer case, but the mouth curling inward is shown on both.

Once you get to the rifling, Harold Vaughn showed 6mm bullets (or maybe it was .270; I'll have to check) 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. It made me wonder if that's the 10,000 psi HP White actually measured as the bullet wasn't really getting into the bore until then. Without looking at their experimental setup in detail, I can't really guess accurately.

December 26, 2012, 04:59 PM	#16
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Slamfire, Yep. Bart, You're thinking static rather than dynamic conditions. When I first joined this board a fellow who had participated in the HP White tests in the 70's said they'd found bullets didn't start moving until about 10,000 psi. QuickLOAD thinks it's more like 3600 psi, corresponding pretty well to the flat spot in Slamfire's pressure curve. The flat spot is due to the pressure increase partly stalling 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 showing 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. Below is a photo of some cases I sectioned to show a well-used verses newer case, but the mouth curling inward is shown on both. Once you get to the rifling, Harold Vaughn showed 6mm bullets (or maybe it was .270; I'll have to check) 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. It made me wonder if that's the 10,000 psi HP White actually measured as the bullet wasn't really getting into the bore until then. Without looking at their experimental setup in detail, I can't really guess accurately. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle Last edited by Unclenick; December 26, 2012 at 05:55 PM.