Hi
sometime it is difficult to find the bullet for clibers like 8mm j or 9,3 and there are plenty of cheap 8mmS bullets or 375 bullets on the market. my question is . is it possible to reseize 375 (9,5 mm) to 366(9,3) or 8mm S( 8,2 mm) to 8mm j (8mm) and what kind of tool is needed?.
if you have done it then please tell me about it.
regards
yes

If you go too far you are at risk of the jacket/core slipping. I have had good results sizing 300gr/.429 bullets to .423 for use as plinkers in my .404j. I size them in a cheap Lee sizing die, first to .427 then to .423. They will shoot 1.5moa @ 100yds which is good enough for practice rounds. Now that's only 0.006", and that is probably close to max.
You'll need a heavy duty press like thr RCBS RC, I use an Ammo Master. You'll also need a good sizing wax like Imperial Sizign Wax & go slow.

Downsizing a few 1/0000s should be okay, but I'm told that squeezing a conventional cup and core bullet down by more than 6-8 1/000dths tends to cause internal distortion that can wreck havoc with accuracy. Just pushing the bullet through the right size hole in a piece of steel will do it, but the "hole" works best if it is tapered, the ram that does the pushing is almost as large as the hole, and the proper lubrication is used.

I've never seen commercially available jacketed bullet resizing tools, so you might have to make/have made your own.

Type the words "bullet swaging" into google and sit back for some good "reads". I have the lee die for swaging .338 down to .329 for my 8x56r that I bought at Dunhams thinking it was a normal 8mm caliber...surprise. It works good but now there are people making the bullet again. Anyhow good luck....I found it's best to stick to the more "normal" calibers whenever possible!!

CH4D sells dies for just this purpose. I think they are special order but are made to fit standard presses and you specify the starting and finishing sizes.

Good luck.
Joe