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.

Last edited by Ken Nelson; 03/22/24 03:54 PM. Reason: Added comment.
Dodging lions and wasting time.....