Steve. It took me a while to find this. The .460 G&A is based on a .404 Jeffery, necked up to .458 and fireformed. The round was developed by Tom Siatos and Jack Lott of GUNS & AMMO. Their purpose was to get more velocity than the .458 Win. Mag. and less than the .460 Weatherby, which they felt was too high.
Apparently the brass is Berdan primed; at least the brass used in the article was. Perhaps some of the suppliers of .404 Jeffery brass now make it with standard boxer primers.
With 500 gr. bullets he got good accuracy with 87.0 gr. IMR4064 and H-4831 at 95.5 gr., although he did work up to 105.0 gr. of H-4831. (IMR-4831 had not been released to the market at this time.) He also said the cartridge would perform well with IMR 3031, 4320, 4350, and similar powders. Max load with IMR-3031 was 91.0 and with IMR-4350 was 105.0 (compressed) These were with the 500 gr. bullet.
Trim to length was 2.86". No over all length was given.
If you can chase down a copy, this data is from the February issue of GUNS & AMMO. 1971.I normally do not give load data on the forums, but I have made an exception as this is an oddball cartridge that is difficult to find anything on. I can't even tell you who is allowed to make you one, as it is a proprietary cartridge, owned, I believe by GUNS & AMMO, and they restrict who can build one. This would make it a very expensive proposition, I'm thinking, but where there is a will, there is a way.
I believe the final velocity decided on was about 2350 FPS.
Anyway, this gives you something to go on.