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Old October 18, 2011, 09:35 PM   #26
Lost Sheep
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Headspace?

Quote:
Originally Posted by inspiron23
I have 2 reloading books. The ABC's of reloading, and Hornady 8th edition. I have read the ABC's of reloading, I didnt quite get the part about the headspace (only the part where it's wrong it can screw up you gun)
I just dont know how it is set by your die. Is it by seating the die (more or less) in your press. I took a few rounds and tried to experiment with the sizing die, but I guess I dont understand how headspace is determined.
Headspace is set by the shoulder on a typical bottlenecked rifle round. Your reloading die (sizing die) can change the shape and location of this shoulder.

The cartridge drops into the chamber until it meets an obstruction. On a rimmed (typically, a revolver cartridge) the rim hits the rear of the chamber/cylinder and the cartridge cannot go any further. This holds the rear of the cartridge firmly against the breechface. Important for containing the pressures inside the cartridge. Brass is too soft to do the job alone without a steel container to assist.

A bottlenecked round typically does not have a rim (the extractor groove may make it LOOK like it has a rim, but if it does not extend past the cartridge diameter, it is not a rim.)

The shape of the shoulder where the bottlenecked cartidge "necks down" hits a matching shape inside the chamber.

Here's where it gets interesting: If, when sizing your cartridge case, you run the cartridge too far up into the die, you set that shoulder back a bit. Then, when you chamber that round, it goes deeper into the chamber. Now there is more clearance between the back of the cartridge and the face of the breech. This is excessive headspace. If there is a WHOLE LOT of headspace, the firing pin might not even reach the primer. But that is not usually the case.

Now it gets not only interesting, but exciting. When this cartridge is fired, the brass expands against the walls of the chamber and friction makes it stick, but the rear of the cartridge is blown back, stretching the brass just in front of the cartridge's base. This makes that section of brass thinner. Eventually (or maybe right away) thin enough to rupture. Rupture at this point lets hot, burning gasses and little pieces of brass (and sometimes steel) loose, inches from your face. (Got shooting glasses?)

OK, it's not as scary as I make it sound. Just don't bump that shoulder back.

There are ways you can check a particular round for excessive headspace, either in your gun or in a case gauge.

I don't know how much of this you already knew from your reading, so I mentioned everything I could think of. I hope you don't mind.

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Old October 19, 2011, 05:48 AM   #27
flashhole
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"OK, it's not as scary as I make it sound. Just don't bump that shoulder back." ... too far. You need to bump it back .001" - .002" to ensure proper chambering.
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