Brass diameter

dalek

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Correct me if I am wrong, but the diameter fro .357 Magnum brass is .379". If that is the case, shouldn't that be the diameter a full length sizing die will bring it to? I have a die that is resizing it to .374" and you can see the difference in the brass
 
I have some freshly resized Winchester brass just behind me..... It measures anywhere from .373- .378 and it's not perfectly round......average is about .375.

Freshly loaded with Hardcast (which should be .001 over) measures about .378, roundness within a thou.

The spec is probably for loaded brass, and I'm sure there is some type of tolerance for it.... say plus or minus .001

At any rate, this explains the need for the expansion die...... You need a snug fit for the bullet to stay in place, and so the powder doesn't leak out around the bullet.
 
Not all brass will have the same wall thickness. (RP brass is rather thin, for instance). For this reason, the sizing die will squeeze the brass down a little smaller than is needed, and the expander will bring the inside diameter where it needs to be for proper neck tension, which is probably .003-.004 smaller than the bullet.

SAAMI spec for .357 magnum is 0.379" (+0, -0.006") diameter for the brass.
 
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Here is an update: I grabbed some commercial ammo this morning, both new and reloads. Both were around .375. I guess I am just being paranoid.

Toprudder, the SAAMI spec is the reason why I blinked. I expected the sizing to bring it to the SAAMI size; when mine didn't I was confused. Glad to know it was just me.
 
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