False Shoulder
A temporary shoulder formed on a case to hold correct headspace during fireforming, keeping the case pressed against the bolt face so it fires reliably.
A false shoulder is a small bump deliberately created on a case neck so the case has a firm contact point in the chamber before it has been fully formed to that chamber. When you fireform brass for a wildcat cartridge or any case with a longer or sharper shoulder than the parent, the unfired case can sit loose, which lets the firing pin push it forward instead of igniting it cleanly. Pushing the neck partway into a sizing die raises a slight step that the chamber catches, holding the case head tight against the bolt.
This false shoulder controls headspace during the first firing, so the primer ignites the charge with the case properly positioned. Once the round fires, the brass blows out to the full chamber dimensions and a true case shoulder forms in the correct place. The false shoulder is consumed in that process and exists only to make fireforming consistent and safe.