Bowhunter - I've done exactly the same thing when I had my 7mm TCU. You simply run the 223 brass into a 7mm TCU sizing die. You load these cases up with a moderate load and fire them in your 7mm TCU chambered weapon.
And unlike the 270 to 6.5 Gibbs above you very seldom have a case split. Brass is very ductile and its actually amazing how much it will shape itself to a chamber.