TH,
Many thanks for your help and encouragement. I really appreciate your effort to post the photo reference from the Fleming book.
I am a firm believer in mag primers for this type of loading whether smokeless or black. My usual "go to" is the Federal 215M and then appropriate use of foam backer rod with smokeless/Nitro for Black; or in the case of black I use any necessary combo of card wads, lubed felt wads, and lube cookies......and NO air space.
Of course Graeme Wright's book is an invaluable guide to safe and successful reloading of these rifles as well.

I have spent quite a while searching the Internet and just found a digitized copy of the J. H. Walsh book "The Modern Sportsmans Gun and Rifle" printed in 1884. Mr. Walsh was an editor of "The Field" and his book is quite revealing.

Here:
https://archive.org/details/modernsportsmans00wals

There is direct mention of ".577 Bore" Eley cartridges loaded with 140-480 (5 Dram) as well as 140-502 and 160-502 (6 Dram).
The stated velocity of the 5 Dram load was 1616'/sec and the 6 Dram load was said to be 1739'/sec, both out of 26" barrels. Nice that!

I have ordered a "Steve Brooks" 480grain paper patch HP mould specifically for this rifle, and "Buffalo Arms" offers .577 solid 500grain swaged lead bullets for paper patching.
- Buffalo also offers 550grain solid lead for paper patch which might be just the ticket or a Nitro for Black work up. (Both companies are in the USA)

You are quite right......recoil with an 8 1/4 lb rifle will be "brisk" to say the least. Still likely not as heavy as my 1902 Webley actioned .577 3" single with 90-650 equivalent. That rifle tips at 11 1/2 lbs and Reloader 15 gives an honest 1950'/sec over my chronograph.
Damn the luck tho, there just aren't any wild buffalo, Tiger, or Gaur living here! (It does knock the stuffing out of water jugs and watermelons....)
Cheers,
- Mike