Thanks Gary, the 338 Win Mag cases that I have are 2 1/2 inches long, if that helps any.
I had to dig out my old Ackley manuals for a look see, and I found four .300 mags in there if anybody is interested. The biggest is nothing but a sharp shouldered .300 Weatherby. There is one that comes off a .348 Winchester case, and looks similar to a .300 WSM.
The number one short magnum (that's what he calls it) is somewhere close to a 2.5" case length, and made off a .300 H&H case (or anyother similar one). But a standard 30-06 barrel cannot be rechamberd to this caliber as the case is a little shorter than the 30-06. With a 150 grain bullet a max load appears to be 64 grains of 4064, and 58 grains of powder for the 180 grain bullet (keep in mind these are for older lots of powder which often is hotter). Max loads with 4350 for either bullet is 67 grains.
The number two mag is longer, and designed to clean up in a 30-06 barrel. He says you can use loading data for the .300 Apex Mag. You now need to get an accurate measurment of the chamber length to see if it actually is longer than 2.494" long. A once fired case would solve all these issues. Ackley does not give the shoulder angle, and it looks like it's about 30 degrees instead of the usual 40.
Being as you have a sizing die, I'd find a half dozen .300 mag cases, and anneal the case necks and shoulders dead soft (your gonna toss them when we're done). Run them thru the die till you are just a little longer than the .308 Norma at the shoulder. Now cut the neck to where they are about 2.50" long. Now I'd start cranking the die down about 1/8th of a turn at a time till the bolt has just a little resistence when fully closed. Now you will know the shoulder gauge line (actually you'll be within a few thousandths, but still plenty close enough). Now make another case that's about 2.56" long (remember we are guessing). Try to close the bolt. If it dose, then you know the case is longer still. But if it dosn't freely close you can trim it .01" at a time till it closes. I figure you will see something like 2.53" long.
The methods discribed are not perfect by any means, but just something to give you a rough idea what you have. Of course you could also have somebody do a chamber cast, and know exactly what you have. Do not reuse the formed brass for loadings as the cases will be way too soft and dangerous (throw them away!)
gary