Does the rifling go all the way to the muzzle, ending in a real crown? If not it's only going to shoot well with sabots.
Otherwise, if it is not counterbored, I have had great luck with the RCBS 450gr. (IIRC) bullet made for the 500 S&W lubed with Bullshop's NASA lube, in my Ruger 77/50, and also the Lee 440gr. S&W bullet and a NEI 500gr. bullet both paper patched. All these shoot better than I can see out to 300M, and mostly all touch at 100M off the bench. The Ruger has a 1:28 twist as well.
You do need to load them stout enough. I am using FFg Diamondback powder, and have to use at least 90gr. to get good bump-up. At 100gr, it's a tack driver, but less than 80gr. and I can't hit the berm twice in a row. Never tried FFFg in it, I imagine you could load lighter but I'd be worried about pressures.