[The_Eccentric]

Mag dimensions suitable for a 6.5 Grendel would likely be a good place to start. The 400L is based on the 6.5 Carcano (which birthed a number of cartridges including the 7.62x39 and 6.5 Grendell) rather than the 30 Remington/10mm auto/6.8 SPC/etc case. Basically, the 400L is a 6.5 Carcano case shortened, necked to .401, and given a rebated rim (.422" vs the original .447") to match a 6.8 SPC/221 Val AR15 bolt face.

The rebated rim takes advantage of the known stronger 6.8 SPC AR15 bolt (vs a Grendell/ARC bolt), and the wider .440" case head/ body makes for an easier feeding cartridge when necked up to .401" (vs the almost straight case of the various 30 Rem/SPC based 400 wildcats such as the 400AR). It also yields a tad more case volume.