Handirifle, No you haven't missed anything, the 8mm is .323 diameter the 375 should be just that ..... .375. That is why I am not willing to invest in the NEF 38-55 barrel, it is a .38, seems most report about .379. A 38 might be period correct, but the Handi is not, nor is 38 correct today, 375 is.
The 8 x57 case and the 444 case are the same length, give or take .010", nothing in chamber. The 8mm would gie up a little length when the body is blown out and the mouth opened up, but again, it would be negligable. The trick here of course is getting a 375 JDJ or close to it, I might even be persuaded to use a 308 case and get a 375 x 308, it would still be a gain of 10 grains capacity over the 375 Win or 38-55 and still function in a Mauser action with no more work than chambering a new barrel.
The 375-06 is very tempting but does need a long action. It would be easier to build one of those on a Savage 110. But it is actually a little more than I want. For that matter a bolt gun is more than I want. Perhaps I need to meditate on finding a Winchester 94 Big Bore. (a joke of course, on second thought, Hmmmmmmm)
Sorry Jeff223, I don't want a 35 Whelen. I'm on a hunt for a 300 grain bullet at 1800-2000 fps that has a better trajectory than the 45-70 offers. Of course I may find the 30-30 Handi is everything I've been looking for, but I do have a pretty serious 375 itch I haven't been able to scratch. I got a 45-70 Handi hoping that would solve it. Nyet.
Edit: a spelling correction