The .577 Grant is an interesting piece; very light weight for a .577 3" BPE at 8lbs 4oz (original post was a typo at 8lbs14oz).
The rifle has a test target fired by Ken Owen back in 2010 with H4198 and a 530grain Minie and he noted that he thought due to the weight that it might have originally been a .577 Snider. Need to see if Atkin, Grant, and Lang want to respond to my enquiry as to any info on this rifle.
My guess is that the regulated 1870's period .577 3" load was one of the light ones with a 480grain HP express bullet and maybe 140-160 grains of powder, though I do not have access to a copy of Fleming's "British Sporting Rifle Cartridges" to check this.
For historical interest I'm going to try starting with a 480grain paper patch HP bullet, 120grains of Swiss 1 1/2 and work up to regulation.
For Nitro for Black I'm going to closely duplicate Ken Owen's load with H4198 and use a 550Paper Patch lead.
Either way, It ought to be interesting.

Basically I think this one is a nice light "stalking rifle" with a man size bore.

- Mike