You would be surprised at what you can form without having "Form dies" by simply starting with the newly chosen caliber seating die and leaving it high up in the press so as when the ram is near the top of the stroke that is the point at which a small amount of sizing begins.. Then merely lower the ram , screw die down a quarter turn , resize , lower ram , screw die down quarter turn etc etc.. When that case is done you continue the steps in the sizing die until you have something that will chamber after it has been trimmed etc.
One thing to keep in mind when forming cases in this manner is that your new cases are going to be quite thick due to the fact that you have been shoving back the shoulder area and case body , so your case capacity will be sometimes significantly less than say off the shelf brass.. Therefore affecting the load data that may be used for that cartridge.
As an example , I have formed a single 243 win case out of a 284 win case using the above method , it's not easy and the brass is merely used as a conversation piece but it can be done.
You will need to do some annealing obviously along with investing in either an inside neck reamer or an appropriate expanding mandrel mates to a quality neck turning setup.
Could it be done? Absolutely , would I do it? No and for two reasons... Firstly the cases would now be using large primers and secondly the diminished case capacity.
Good luck.
