375 Winchester cases are quite a bit thicker through the web area and up the side of the case a ways. They were built for higher pressures than a 30-30. You can form 30-30's out of 375 Winchester cases. Have you ever reformed cases? If you have then you know what you would be getting into. They only way I would reform cases again is if
1. I could not get cases in what I was shooting - an obsolete cartridge or a wildcat.
2. I wanted some thing special like taking 223 Lapua brass and forming it to 221 FB because Remington is the only supplier of 221 and I want a better quality case for guilt edge performance. The trade off is a lot of work, higher cost, sizing, trimming, reaming, annealing, neck turning, ect
3. If I had 5000 375 Winchester cases and had more time than sense. Sell the 375 Winchester brass and buy 30-30; 30-30 cases are much cheaper and a whole lot easier.
The bulging would happen even in a thicker walled case. To minimize this effect; only neck size and leave the bulge alone. Only full length size when the bulge gets to the point of difficult extraction.