I am sure this has been done but wondering if it ever had a following or offered any type of improvement over the standard 404 Jeffery. Take the standard 375 H&H and open it up to.423. I suppose it would be similar to the 416 Hoffman, just a little bigger.

70,
What you describe is or close to the 425 Express. This case can be made from blown out 300WinMag cases, which will have the "right" neck dimensions, or a necked down 458Win case, which will have a neck about 1/16" short. I'm lazy enough to neck down 458's.

This will push a 400gr bullet close to 2400fps according to some sources. My shoulder and attached constitution limit my loads to 2340fps. This is using H4895 and a 400gr A Square Lion Load. A Square developed this load and made brass for it.

My rifle is a Savage Safari which has a muzzle brake that can be turned on and off and feeds from a straight stack blind magazine. The load above will print with iron sights about 1" a 100yds. Kudude

you're talking about something half way between the 416 Rem Mag and the 458 Lott.

I don't think it'll have as much capacity as the current 404 Jeffery.

Interesting idea and I'm sure it's been done. The song just isn't moving I suspect.