i.e. a necked down .416 Rigby?

Actually the 338 Lapua is a shortened Rigby and with a more slopy shoulder for feeding. Thus with the sloping shoulder it has less capacity than if it was just a shortened the Rigby.

The idea behind the case is to get the required case capacity from a cartridge that is not overly long and at the same time being a cartrdige with taper and slopy shoulder. The 338 RUM is about the came capacity but a bit longer and does not have the taper. Although the 338 RM is shorter than the 300 RUM, 300 Wby case etc.