I have a similar problem with a bunch of Berry's Bullets .459"/405-grain hardcast.
They are super in the tighter-grooved .45-70 Gov't. SAAMI minimum groove for that is .456", and .457-458" is a common in-spec barrel.
I had poor accuracy with that bullet in a .459"-grooved .458 WM at faster than 1400 fps,
but I found that if I kept it under 1400 fps with AA-5744 (no filler needed),
or whatever slower smokeless powder with a filler if needed to get it below 1400 fps,
it shot close to 1 MOA for 3-shot group.
Give that a try before you give up on your .459"/465-gr hardcast.
Saint Bagwell could kill anything that walks with a 480-grain Lyman PH of relatively softcast (50:50 WW&Lead) at under 1300 fps MV.
Speaking of giving up, well, I am just going to stop dithering with the 450-gr TSX.
Sumbuddy may try something else like a packed case of droptubed H335, but not me.
I doubt I will ever do anything better than the following:
I will probably never improve on this either:
I reckon 83 grains of AA-2460 will be perfect with the 480-grain XTSX from Bubba.
And I have a bunch of North Fork 450-grainers, SP softs and FN solids, that should be perfect with 82 grains of AA-2230.
Now the question is, will I use compressed H4895 or non-compressed AA-2230 with the 400-gr XTSX from Bubba ?
But first I must do the duplexed BP and paper patch as previously laid out,
and then the single ball in sabot and the shot capsule loads with Green Dot and Blue Dot respectively.
I will be deer hunting this year with the .461"/ 579-gr FNGC "Big Blue" hardcast at 3.340" COL,
propelled by 41.5 grains of Blackhorn 209 with filler (+/- 1300 fps) and using the open "express sights"
on my .458 WM Cabela's 50th Anniversary Winchester M70 Super Grade.
She came in a replica 1961 cardboard box, from the Louisville, KY Cabela's, in 2013.
Price was reduced after she waited two years for me to happen upon her soon as they opened the store in Loo-uh-vull.
It was love at first sight.
That load is zeroed at 85 yards with the low leaf sight, windage perfect, just as she came from the factory.
I might eventually put the Near 25-MOA Picatinny on her, and a vernier sight on the heel of stock.
She will stay under 10 pounds like that, might need to get a trigger job to make the 3-pound minimum trigger pull.
That 579-grainer at 1300 fps will make a dandy gong ringer out to 600 yards, looking for 1000.