Ah, another topic near and dear to my heart. First, to clear up a little confusion: it's the .38-55 that's the longer case. The .375 has thicker brass and holds less powder. The .38-55 came first, the .30-30 was derived from the earlier .38-55. The .375 was developed in the 1970s from special thick walled .30-30 cases. So... it's the .38-55 that's the daddy and grand daddy of them all. (BTW the .30-30 was never a BP cartridge, it was smokeless from day one, the case will hold way more than 30grs of BP). There are minor differences in bullet IDs. The .375 Win is a true (nominal) 0.3750". The .38-55 is anywhere from 0.3755" to 0.3800", you just have to slug the barrel. The new Marlin is 0.3755".
The two cartridges are so similar that most actions will chamber both. The new .38-55 Marlin will chamber and shoot .375 Winchester with no problem. My .375 Win T/C Contender will chamber and shoot .38-55.
CAUTION: Vintage firearms chambered in .38-55 will chamber .375 Winchester cartridges, however you should
NEVER fire factory loaded .375 Win in pre W.W.II guns because they can't handle the higher pressure. .375 Win
brass could be used to work up light loads for an old rifle, be safe - keep pressures down and use your noggin.
Historically, the .38-55 Marlin-Ballard was the daring of the 19th Century target shooting crowd. Easily the most accurate black powder cartridge ever developed inside of 250 yds. Sort of the 1890s version of the 6mm PPC. There were many very fine single shot "Shutzen" rifles made in .38-55 which are today highly prized by collectors and shooters. -- Kernel
A gent named Paco Kelly, who belongs to a group called THE SHOOTIST, has written a lot of interesting articles about this topic. Below are the links, cached on Google Search:
THE 38-55 IS BACK
38-55 AND 375 POWERHOUSES
SLEEPERS IN POWER
[This message has been edited by Kernel (edited February 25, 2000).]