Well, if you are willing to extend that to 'nitro-for-black' loadings, the rest of the board here are sick of hearing my opinion!

Marrakai's Opinion!

The .577 x 2 3/4 Light Nitro is a BLAST!

If the barrels are 26 or 28 inch, I recommend starting at ~90 gr Varget beneath the 650gr Woodleigh RNSN (Cat.No. 5 for BP Express), with a magnum primer, and just enough felt wadding or dacron to slightly compress the load. Use a chrono and work up to 1650 fps or thereabouts, looking for barrel regulation. That velocity should be achieved with less than ~8 tons of pressure, well below the 10 tons generated by the BP Express load with only a 570 gr lead bullet.

(all the usual warnings, start 10% below, no liability, etc, etc... )

If you want to shoot lead bullets, you need go no further than Sherman Bell's recommendations in the Spring 2005 DGJ. He brought a Greener hammerless double chambered for the 2 3/4 .577 Express out to Australia, and hunted buffalo with ol'mate Dave L_. He used 570 gr 'long flat nose' lead bullets from an LBT mold, loaded over 70 gr IMR4198 with 8 gr dacron wadding, and they produced 1700 fps MV and 3 1/4 inch groups at 100 yds (with express sights). Worked well on buffalo, too!