I'm hoping to locate dies for the 25 Souper. Failing that, I wonder if the round could be loaded with some combination of common dies or if a 308 sizing die could modified with a bushing to reduce the neck diameter. ( Why do I get involved in projects like this? )

I’d try running an expander mandrel of appropriate diameter into a sized .243 case. Necking down .308 brass may provide a too thick neck. Maybe neck down a .260 ??

My shooting buddy had a .25 Souper many moons ago. He never had specific dies for the gun. I’m sure he started with .243 brass but I can’t remember how he achieved sizing or seating. He was a machinist so probably just made or modified existing dies.

I prefer fireforming without a bullet, in cases where the donor case is already close to the final length. My previous experience, with the common expander button in common loading dies, shows that significant trimming to "square up" the neck is often required. Expanding a .243 neck to .257 will already shorten a little bit, so unnecessary additional trimming should be avoided. I think I would try sizing a 308 in a 250-3000 die until it just chambers and fireform it w/o bullet. then if the neck in too thick, I would decide whether turning the necks or switching to 260 is better overall. I have plenty 308 brass, but not much 260 or 243. I don't have much trouble with thick necks unless I start with donor cases (like 30-06) that have to be shortened well into the body area. I suspect 250-3000/250 Savage dies can be used to load the .25 Super. They should only need to be carefully adjusted (after smoking the case neck/shoulder) to not set the shoulder back. Buying a new set of 250 dies would likely be cheaper than having custom 25 super dies made, if you don't already have them.
Mike

Use a 260 rem full length die with a neck bushing. Use the appropriate neck bushing for the 25 caliber. We do this all the time for the 20 practical. Just use a 223 die with a smaller neck bushing. I like the Redding type S die.

John

Thanks for your very helpful suggestions. It all sounds easier than I thought. A couple more questions about the bushings: Does the seating die take one too? Can I deal directly with Redding or should I go through a dealer?

You could probably just use the 260 seater. Or open up a 243 seater. Midway of Grafs probably have the .260 die and assorted bushings in stock.