In wildcatting and reloading lingo, necking up is the act of taking an ammunition case, and by the use of the appropriate sizing die enlarging the case mouth to accept a bigger bullet than it was originally desgned to. See .35 Whelen.

This could only be practically done on cartridges that had a shoulder, or bottlenecked. And the necked up mouth size can not be larger than the base of the case.Straight wall cartridges would look extremely awkward, not to mention probably will not feed reliably, if necked up.

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