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257 Ackley Short Magnum

His book only says 300 win mag necked down and shortened. Obviously you asked for case dimensions but why do you ask?
I’ve purchased a gun that was built by an older gunsmith who passed about 10 years ago. Only said 257 on the tag and was assumed by the person settling the estate to be a 257 Roberts. It’s a belted magnum that pushes a .257 pill down range but cartridge is unknown. He made his own barrel. I’ve made a couple wax castings and was trying to figure it out. Taking it to a gunsmith Monday for diagnosis.