Nrut,
I have a smooth bullet. Since I had to order the mold custom-made and a .350" bullet would be too small for regular cast bullet applications I just had the lube grooves omitted.
However, you're in luck with that 9.3mm! You can probably buy any .358 mold on the market and patch it up to .368". If .368" won't chamber sizing it down to .366" would be a cinch.
Don't worry about the lube grooves - you can patch a lube-grooved bullet just as well.
I use straight wheel-weight alloy, air cooled. However, for your application, you might want to use plumbers lead. See I'm guessing that your 300 gr NEI will cast .366 diameter bullets or greater. Then you'll add another 0.010" of paper to that diameter. That gets a might tight to squeeze into a chamber, let alone through the bore.
There's two ways around your dilemna. The first one I know will work. Use plumbers lead to cast your bullets. Patch 'em, then size 'em to 9.3mm. The soft lead will size easy enough that it won't destroy too many patches. If you use wheel weight (WW) alloy you'll destroy most of the patches.
The second possibility is to cast your 300 grain bullets with WW alloy, then size 'em down to .358". I don't know how practical this is - that's quite a squeeze - but then, you need not worry about schmeering the lube grooves shut.
I've patched bullets for my .357 magnum using .358" grooved molds. I just used plumbers lead and sized 'em down after patching.
I haven't used plumbers lead in my .35 Whelen mainly because if I can get WW alloy to work I'd rather use that and save my pure lead for muzzle loaders. Though it doesn't cost much more, pure lead scrap is just more difficult to come by.
Do I plan on hunting with my PP bullets? Oh heck yeah! That's my main purpose for getting into PP. I have absolutely no interest in a bullet that is so hard it doesn't expand on game.