Easiest way to make .250 brass might be to use the "cream of wheat" method.
Load ~7gr of Unique in .22-250 case and tamp with cream-of-wheat or "grits".
cover with a piece of toilet paper to keep cow or grits out of your reciever/action.
Aim in safe direction an pull trigger. Case will come out expanded to fit your chamber.
You can also probably just use your decapper/expander to expand the case neck and do the same thing. If you can't get the decapper to enter the case neck without hanging (ie; some Lyman, RCBS, ect dies), you can use a small screw driver inserted in case neck to flare the case mouth by "wallowing" the screw driver around the perimeter of the case mouth.
It's not "rocket science"...
Using a .30/06 or 7x57 or such case would be the hardest way to go about it.
I've used my Lee sizer dies to make .257Roberts cases from 6mmRemington cases. Just lube the case, brush the case mouth and run it through the die. Presto, a .257Roberts case.... Roughly the same as expanding the .22-250 to a .250..