When I needed some, I took a 32-40 sizing die and trimmed a little off the end until it had the right base diameter.  It worked well.
Bruce

Mike, have you checked the case capacity of your cases, against mine?

I do mine a little different and only swage down to the solid head and not past it, before machining.

I wouldn't mix the cases until they are checked, as mine are marked FC 30-30, too.

Does rolling them to size shrink the primer pockets? Would like to know how the "rolling" is done...

I place a case between mandrels. The mouth of the case is held in the lathe chuck and the other, primer pocket is held on the other mandrel which turns on a live center.  Making sense? I had taken a bunch of pictures but can't find them.

A small three wheel rolling mill with a hand crank would work better. I did this last winter just to see if it would work and it did. Once rolled to size the head is concave. This is where knuckle dragging cave man experience does it's job. I place a .310 drill rod in the case, place it on a solid flat surface and give it a whack with a hammer. That flattens the head back out. The primer pocket stays the same diameter.

Joe,
This looks like a VERY practical approach to make 25-20SS from .223 brass and perhaps 28-30 from 30-06