It usually surprises me how fundamentally ignorant the shooting public is. Most people never really understand why or how guns go bang. They don’t understand what a case does, how it works, or really anything else beyond the operation of the firearm, and even that people get wrong.
Here is a demonstration of the difference between a 30-30 case and 375 Win. These cases are both based off the 38-55 case from the 19th century. In fact, a 375 case looks so similar to a 38-55 that it is almost impossible to tell them apart visually, but what is inside may be different.
The 38-55 was designed to work at black powder pressures, and the brass was thin. Both 30-30 (1895) and 375 Win (1978) are modern cases, designed in the era of smokeless propellant and were never factory loaded with black powder. The 30-30 maximum operation pressure is 42,000 PSI and the 375 Win is 50,000 PSI. That extra pressure in the 375 Win is held back by extra brass in the case head and web area. Modern 38-55 Brass (this is from Starline) is about as thick as 30-30. And it is STRONG brass. Much harder to cut than the Winchester made brass (one of the advantages of using hand tools).
Due to the present-day scarcity of 375 brass many folks are using trimmed down 38-55 or 30-30 brass that has been necked up. If you are doing these things adjust your power levels downward using starting loads of 375 Winchester as a max or using 38-55 Data! The truth is that 38-55 has plenty of power and a flat enough trajectory for deer hunting east of the Mississippi. And the lighter loads will be more pleasant to shoot and everything will last longer. 375 Win, to my thinking, was doomed by the fact that it was too powerful. I really think that if the folks at Winchester in the late 70s had capped the 375 Win to 45,000 PSI they could have used the same brass coins and most of the same dies they used to make 30-30. And there would have been no need to strengthen the actions of popular 30-30 Lever-actions like the Marlin 336 and Winchester 94. It would have just been a straight forward different chamber offering. Nowadays the 375 Winchester is mainly enjoyed by Contender shooters. It is an excellent cartridge for a Contender offering ample power and is quite efficient in shorter barrels. Since Contenders have no magazine, the bullet can be seated way out and pointy bullets may be used.