Like he said .250 Savage. But why would you "need" to make up brass for a 22-250. It is much easier to find than .250 Savage brass and doesn't require necking up or down...just load and go.
With the right amount of effort you could make 22-250 brass from nearly anything with a .473" rimless head size. .243, .260, 7-08, .308, 30-06,.270 and on and on and on. Better get out the Imperial tho.