How to modify a cartridge spec to create a WildCat?

I'm making a wildcat 9mm AR round. I've trimmed a .223 case to eliminate the shoulder and intend to load it with 9mm projectiles.
How to create a wildcat cartridge in GRT?

Some thoughts: The 223 case is smaller than a 9×19 mm case, the wall thickness is in our opinion too thin at the mouth: .355“ + 2 x .012” = .379“. Most 223 / 5.56 case base dia. = .373”. So neck thickness can only be .008“ giving a little expansion to release the bullet. .008” neck thickness will not hold the bullet securely, and the bullets get set back during chambering. It could work if you can find some SAAMI maximum casings, maybe Starline basic brass, or you expand the whole case to the base with a mandrel. That would make it a 350 Legend case clone.

First of all, whenever you want to know the meaning of any symbol used in the drawings, please take a look onto this page and scroll all the way down to the explanation of the symbols used in the cartridge designer and drawings.

• 1) Open the caliber database and select the parent case you want to modify, in your case .223 Remington:
  
• 2) Right click and choose “New with a copy..”:
  
• 3) In the opening window enter your cartridge name and alternate name, then just click OK:
  
• 4) Your new cartridge will show up in the user database tab:
  
• 5) Right click onto that file and choose “Edit in Cartridge Designer..”:
  
• 6) Now you can edit your cartridge with a realtime drawing:
  
• 7) You want to just remove the shoulder, so click on “L1” of both, the chamber and casing in the drawing or list and remove the corresponding value:
  
• 8) Same for “L2”, “r1min”, “r1max”, “r2”, “H1”, “P2” and “alpha” - that's basically the shoulder. Now it should look funny like that:
  
• 9) Now you have to modify the diameter of “H2” to the outside diameter of your case mouth. e.g. the value from “P1” (bottom diameter = straight) for the case mouth and in the chamber copy also the value of “P1” to “H2”:
  
• 10) You now have to copy the round diameter and the barrel specs of e.g. 9×19 into the corresponding fields of “G1”,“F”,“Z”,“b”,“N”,“u”,“Q”,“i”,“alpha1”, etc., and remove “h” and “s”:
  
• 11) The last thing to do is to let GRT calculate the approximated fired case volume by clicking on the calculator icon next to the case volume entry in “Cartridge” tab:
  
• done

Note: The created cartridge here does not make much sense. This is just an example of how to do that.