Josh,
Over the years I have owned three .458x2's. I still own the last one: A M-700 nee; .350 Rem Mag Classic, rebarreled by McGowan. It was built back in the early 90's by a friend of mine. When he passed away, I purchased it from his estate. I have not fired this one.
The 1st one I owned was a M-600 in a MPI stock. It kicked like a Missouri mule. I didn't keep it long. Truly you are correct, there is not much data available.
The .450 Marlin is virtually identical with the exception of a wider belt. (Thus it cannot be chambered in other magnum rifles...) I would not hesitate to start with data for the .450 Marlin. I'd work up loads very carefully, and stop when chronograph readings are reasonable.
There is NO need to try to make it a .458 win Mag. If you need more power than the 2" case will provide, then for goodness sake, buy a .458 Winnie.
An astute handloader and shooter, will likely reach his recoil tolerance well before maximum loads are acheived. My best advice is to pick a reasonable velocity goal, and stop when it is reached. Powerwise it is slightly better than a hot loaded .45-70. It's advantage is that it lies in a short, handy, bolt gun.
I see no reason to use bullets heavier than 300-350 grs. The heaviest bullets are designed for use in the .458, anyway. The
Handloader's Manual of Cartridge Conversions lists only one load:
40 grs. of IMR-4198 with a 350 gr. bullet for a velocity of 1825 f.p.s.
More data is listed in CoTW by Frank Barnes. That book is an invaluable reference to us rifle looneys. Ironically, it was Frank Barnes who initially wildcatted the .458x2. If you don't have that book at your disposal, I can Xerox that page and snail-mail it to you...
My buddy shot at least one deer with the cartridge. He said they dropped like they were struck by the hammer of Thor! I always thought that one day, I might carry his old gun out and take a deer in his memory...
Hope this helps...
Grasshopper
