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Old December 20, 2012, 07:21 PM   #22
Gerry
Senior Member
 
Join Date: December 24, 2010
Location: Canada
Posts: 382
I don't know why folks are advocating crimp to prevent set back. Set back happens because you don't have enough tension between the bullet and your case. In a straight walled cartridge like 9mm, this tension is achieved by pressing in a bullet that is slightly too big for the case. What holds your bullet is only this tension of the case neck against the sides of the bullet itself. The amount of force to press the bullet in further would continue to deform the case walls by stretching it. This takes a lot of force! Hence the press to get the bullet into the case in the first place.

A taper crimp overdone can undo this tension. The reason is that copper springs back more than lead does (even using jacketed bullets). You squeeze the bullet a bit inside the case, and the case later springs back a microscopic amount while the bullet itself does not to this extent. This reduces case tension. The taper crimp die should only be used to remove the bell created in order to easily place the bullet on the case mouth prior to seating it.

I use beveled lead 9mm bullets and do not bother crimping at all since I don't need to bell larger than the bullet diameter for easy seating. They work nice.
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