243 Src brass forming

[Waknstak]

Recently acquired a new barrel. On this chambering and I have never formed brass..... I have read that it is a 2506 necked down and blown out. What does blown out mean? Does this blown out process happen the first time you fire the brass or is it a different process altogether? Also I have heard that you can form the brass from the 270 case which might work better in my situation as I have a 25-06 it not a 270 so maybe would prevent accidental wrong ammo situation.... Could someone explain the process of forming this brass or I would be willing to purchase some of someone has a bunch laying around..... Thanks