.33 Winchester and .375 JDJ aren’t straight wall cartridges. Much easier to take those down.
But taking a straight wall cartridge down to a smaller taper and maintaining straight walks seems like it would be harder. I have never tried it though.
The aren't straight but their parent cases started out straight. At the end of the day, a long tapered wall is just a 'shoulder' by another name. Or a series of shoulders. And you can always slightly over-form them then fire-form them back to fit a tapered chamber.
Don't get me wrong, I've never done this - I just know that brass can be worked an insane amount if you lube it and anneal it and take small steps.
I went out and dug around in my junk box and found an artifact from my 6.5RM project - that's a .375H&H case, a 7mmRM case, and a reformed 7mmRM case where I'd chopped off the neck at the neck/shoulder junction, then did an intermediate step with a .350RM resizing die then a cut-off .300WM resizing die then a 6.5RM resizing die. Note how the shoulder of the original case was turned into a half-inch of long skinny neck, without the walls collapsing. All I'd have needed to done to 'finish' this case would have been to cut the neck off at the correct length (or cut it long and then trim to length and chamfer. With some rifles and some brass I would have needed to ream or turn the necks (for example, you can't use Norma brass for case forming smaller stuff without turning the necks, the walls are too thick) but with this brass, it wasn't needed. Just trim, load, and shoot, and for some reason that case never got finished when I sold the rifle. But notice how long the cylindrical portion is. Compare that to the .375 parent case, which, incidentally, will almost hold a .355" bullet tightly as-is in its unfired state, and if you'll notice, has almost zero shoulder:
It would be dirt simple to reform that .375 case. You could probably do it in a single step, or no more than two steps, with the first being an intermediate step that added more taper to the entire case body and the second finishing the taper and leaving a short neck to hold a .358 to .375 caliber bullet.
The advantage to such a rifle, in this case, is that you can actually *find* .375H&H brass.
If brass was available, you could also start with the .375 Flanged Nitro Express 2.5" (note that the 2.5" case is a different case altogether from the standard .375 Flanged Nitro Express) and if you had modern brass and a rifle that could handle 65kpsi ammo, you could very easily turn such a thing into a legit 400-yard cartridge. The .375H&H reformed to straight-wall should be even easier, because you could do it at lower pressures.
I mean, recoil is gonna be serious either way. But the OP already knew that.
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