There was a 338 Talbot built with BMG brass..Ed

It would be a fun experiment, but I don't think you could use all of the room that case would give. The Talbott was cut down quite a bit. Maybe with a really long tube (40"?) you could get something of value that you couldn't get from, say a full-length Gibbs (Chey-Tac) case.
Also, have you seen the thing? Hard to find an action for that monster casehead!
If you give it the same length-dimensions as the Lapua, and improve the taper to something more like a sporting cartridge (rather than a frikkin' artillary round) you have almost double the powder room the Lapua has!!! And that puppy already needs heavy bullets and a 30" tube to make any sense (to me.)
Funny, looking at a sketch of this cut-off, necked-down case, it looks proportionately like a 270 WSM.
"Honey, I blew up the short mag!"

The .50 cal has been necked down to various things before. Results have never been very spectacular with anything and barrel life is PITIFUL. It's your money.