Thread: Resizing issue
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Old October 26, 2012, 06:38 AM   #30
Bart B.
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Join Date: February 15, 2009
Posts: 8,927
Guffey, force equals pressure per square inch multiplied by the area it presses against. The inside of a large rifle primer cup at it's bottom is about .0191 square inches (.156" diameter for a Federal 210, squared times .7854). 1360 pounds per square inch multiplied by .0191 square inch equals 26 pounds of force.

I forgot to subtract the .027" cup thickness Federal large rifle primers have so pressure needed's 360 more than 1000 pounds per square inch I first stated. And that pressure presses the sides of the cup against the pocket wall, too.

Note also that the .308 case was driven hard against the chamber shoulder from firing pin impact and there's a thousandth or more case head clearance to the bolt face. The force of the primer's explosion on the pocket's bottom ain't enough to drive the case any more forward. And the .045" diameter flash hole lets some of that energy go into the case body which will reduce the presure in the primer cup by some amount I don't know how to calculate. Verified this by firing several primed cases without powder or bullet measuring their case headspace before and after each time. Average shortening of this measurement was about .0015". And not one single primer backed out of the case; they were all about .002" below flush with the case head.

With a live round, its body stretched back until the case head stopped against the bolt face drawing brass back out of the neck and up the shoulder therefore shortening the case as it expanded against the chamber wall. This is why the case was shorter from head to mouth after it was fired. When the diameters of a fired case are larger than before it was fired, the brass mass remains the same so the case ends up shorter. The reverse happens when the case is full length sized; brass gets moved forward as the case diameters are reduced. Most folks believing firing stretches a case do thinking the case head's held hard agains the bolt face by the extractor and stays there when it's fired. This only happens when bottleneck cases headspacing on their shoulders fit the chamber very tight with zero head and shoulder clearance; the bolt typically binds hard when closed on such a case.

A friend got 56 reloads on his Federal case full length sized in a die a lapped its neck out. His rifle was clamped in his free-recoil machine rest and the 57-shot group on the 100 yard target was about 3/8ths inch center to center between widest shot holes.

Last edited by Bart B.; October 26, 2012 at 01:59 PM.
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