I've acquired a rifle chambered for a 250 Magnum, as stamped on the barrel.
It is also identified on paperwork as being chambered for the 257 Ackley Magnum. I can find reference to the 25 Ackley Magnum,..are these the same cartridges?
I'd like to locate a set of dies,..or I'll have to investigate rechembering to 257 Wtby...

Like you I see a reference to a 25 Ackley Magnum in QL. But no diagram. They call the capacity as 75grs which would indicate something largeer than an 06 case.

Of course there was the 250 Savage AI. The 257Roberts AI. Etc.
I would suggest you start with a chamber cast or if you have fired case have custom dies made.

do a chamber cast

it's clearly a belted case...I think it's this one..

http://www.loaddata.com/member...r%20Reloading%20Data

With all the recent interest in the .25ACP, maybe a belted .25ACP?

quote:

it's clearly a belted case...I think it's this one

If so it is pretty much a 257-264wmag. Shoulder is moved back a touch and the case is a touch shorter. Like I said QL calls the capacity as 76grs compared to a 264 of 82grs. To me it makes more sense than 84gr 257Wby case.

RCBS lists the 250 Ackley Magnum on their specialty die list. I would still do a chamber cast to be sure. Hornady will make you custom dies for the price of RCBS special order dies. Then you would be sure it matches your chamber.