8x57: Good for you! I was thinking of you the other day when I saw that 8mm Karibiner mold I tried sending you sitting in on the loading bench. Did you finally get a standard Lee .323" mold?? I'm glad you have taken the plunge. It is a wee bit addictive, especially when you DO realize that you're practicing for pennies and keeping your barrel in top shape.
I have had a heckl of a time trying to get ramped up for the 9.3x57. I ordered a mold from N.E.I here in the states: a 285 grain GC mold, .268" diameter. A standard catalogue item. What they sent me was a 285 grain plain base that was .264" diameter... and then had the gall to tell me it was what I ordered. I was out $110 + 25 shipping. I considered wrapping it with a paper patch and made a few trial bullets which wrapped up to .270" but I haven't shot them.
As to your question: Yes, it's possible to progressively size .375 down for the 9,3 but unless you have a reallty wide groove diameter, sizing all the way down to .268 is bound to damage accuracy. I agree with you about how Lee should make a mold for this caliber. A friend and I have been trying to gin up some intertest in a "group buy" on a custom mold from Lee. Lee will make you a custom mold 6 cavity mold for $55 US + a $100 set-up fee. After that, all the rest of the molds cut thereafter will be standard mold cost. I had a custom .303 mold made that was .317" in diameter with a .307" nose and it took them about 6 months. (the quality is excellent) A 9.3 mold built on the model of their .338 mold but with larger dimensions would weigh in around 265 grains, I think.
Lee will make custom sizing dies for $25 US each. A proper set would run you about $75 US, I guess. What US shooters are doing is taking the relatively common .375 jacketed bullets and running them through a set of Lee sizing dies in steps down to .266 (or whatever). Lee is aware of the practice and will ask you if you are sizing jacketed bullets and will alter the bearing surface to accomodate for no extra charge. The results are excellent, from all accounts.
What kind of 9.3 are you shooting? I have a 1940 Husqvarna ( 98 ) mauser in 9,3x57. I have yet to shoot it.
In any event, if you buy a mold, steer clear of NEI!! THey screwed me. ~Muir