necking up

[tapalmer99]

I have heard the terms "necking-up & necking-down" several times, but don not understand what the purpose is, and am unclear on how to do it.

Can someone give me a brief explanation on how, and why, it is done?

Thanks.

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[KRP]

Necking up is enlarging the case neck to accept a larger diameter bullet, necking down is just the opposite. Allows use of different caliber with a certain case. Sometimes it is as easy as a single pass through a die with the smaller/larger neck, sometimes it needs to be done in several steps.

Some common examples:

338/300RUM(Edge) is the 300RUM case necked up to 338 caliber.

6.5-284 is the 284Win case necked down to 6.5(264) caliber.

 

[tapalmer99]

Does the bullet diameter need to stay smaller than the rim diameter? What would the rules be?

(Thanks for such a quick response!)

 

[KRP]

Does the bullet diameter need to stay smaller than the rim diameter? What would the rules be?

(Thanks for such a quick response!)

Yes, for all practical purposes, otherwise the neck would be larger than the base and the cartridge won't chamber.