Wiktionary
neck up
vb. (context firearms English) to increase the size of a cartridge's chamber and calibre.
Usage examples of "neck up".
Without Perrin about for her to try to impress, her dark blue riding dress was cut quite modestly, with a neck up under her chin.
Not a bad trade: From the neck up, she looked older than her forty or so years.
But she seemed to find nothing satisfactory and flailed her hands around wildly, craning her neck up to keep her mouth away from.