Here's a rough and ready way to form .30 Luger cases from .223 / 5.56mm.
Make a wooden jig to allow you to hacksaw .223 cases to an appropriate length. Deburr the shortened case with a file, and then run it through a .30 Luger trim die. You will need an extended shell holder to do this. The trim die has the same contour as a sizing die, and will easily form the shoulder and neck. File to length while it is in the die. If you got lucky making your jig, there won't be much filing to do. You now have a .30 Luger case with a neck that is way too thick.
Place the case in a 9mm shell holder from a Lee trim gauge, crank it down tight (a vise and pliers are handy here), and fasten the spud in a drill press vise. Hold the vise loosely and position it so that an 8mm reamer in the drill press will self center. Advance slowly until the entire neck is reamed. Remove case, knock out the shavings, and chamfer.
Run through your usual sizing/expanding dies, it's then ready to load - but will have less internal capacity than commercial brass. The ones I made from PPU .223 had about 6% less, so reduce your charge accordingly.
.223 cases are slightly smaller than .30 Luger, so the fired case will look a little pudgy were it expands above the base.
There are other and better ways to form cases, but this let me do it with an 8mm reamer ($9 from eBay) as my only expense. I had fun, but won't be doing this anymore unless the commercial brass supply runs dry.