I also have tried making thicker 30-30 cases from 375 winchester cases. Just be sure you use the greasy stuff in the little round tin. Stuff works wonders and is way better than the regular lube some of us use when either forming or sizing cases. Dug through my stash of odd ball dies and came up with a 30-30 form and trim die. Set it so the base of the die does not contact the top of the shellholder. Lube the case with the sticky stuff and resize only about half the case. Raise up the press handle and screw in the die about a 1/16 turn then rotate case 180 degrees then finish resizing. Try the case to see if it will fit into your chamber. If not rotate the sizing die about 1/16" and repeat the process. Cases may come out a little short. I used a lee handtrimmer to trim cases to the same length. Take one case and seat a bullet either cast or jacketed and see if you can chamber either. If you can't then you may have to get an inside case reamer. I believe the forster company still sells the neck reamers for use in their version of their case neck trimmer.Try and use a ball mine to get an idea of how thick your brass is at the neck. I used the inside reamer as I already had it on hand. Lube the reamer when reaming. removes metal faster and leaves a cleaner cut surface.Now if using cast bullets just make sure you have enough radial clearance around the case neck to release the bullet without raising pressures. And remember when using the reformed brass it will be thicker and heavier that the origional 30-30 case. Less powder is needed, due to less internal volume. Took awhile to get my fifty cases done. If you luck out and latch on some once fired cases they will reform, but takes a bit much more oomph. New unfired cases were the easiest.I wouldn't waste valuable powder fireforming just the cases. Buy some remington 170 grain round nosed softpoints and use them to fireform. Remember when forming the cases you want to be able to feel the case being pushed into the chamber. It might be felt as some resistance and as long as it chambers all is well and good. It will form to your chamber when the big light is lit. Just don't make your fireforming loads loadbook maximum. About halfway is good enough. Hope this helps. Frank