These neck size bushings are specifically for the Forster Bushing Bump Neck Sizing Die
If you’re making cartridges for a tight chambered rifle, you might need smaller bushings. Alternatively, you might need larger bushings if you are not outside neck turning and are using brass cases with thick neck walls. Most sizes are available in .001″ increments; just order by the inside diameter desired.
You can adjust the bushing height within the die to allow partial case neck sizing, allowing the unsized section to better align the cartridge in the rifle chamber.