Well, you may be thinking of Saeed's 404 necked down to .375"?
Scroll to the very bottom, click the link that says "accuratereloading.com" on the lower right, and on the front page at the bottom you'll see "reloding" or something like that.
That takes you to the front page of the relad data section. Saeed's 'cat is on there, under 375 cal.
An H&H necked to .423" is a very interesting proposition.
No reason it couldn't do just a hair better than the 416 Remington with 400g bullets.
Selection is severely limited, I believe, but kinda sounds like a neat idea.
Look at the 416R for load data. You could use those same start loads, and expect just a hair better peak bullet speeds with same-weight bullets, as is generally the case with same case, bigger bullet.
Has something to do with bearing surfaces, expansion ratio, harmonic convergence, etc.