What 'company' are you talking about? It sounds like you are trying to make a choice between magazine options this 'company' offers based on parent case
The EDGE RUM case is longer than the 338 Lap or 300WM cases and you will need a longer box than what would be made for them IF you are trying to maximize case capacity and use long bullets for long range work. The RUM case is between the Lap case (larger case dia. than RUM) and the 300 WM in terms of case dia. What does the company say when you ask them which would work for your RUM application?
In terms of long mag boxes, if you are talking about a Rem 700 action or clone the longest box I know of:
CFE-9 Magazine Box, Wyatt's Outdoor
I've got two of 'em. One in a 300 RUM in a Rem 700 action and one in a 338 Lap Imp on BAT HRPIC action.
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